New insights into enhanced anaerobic degradation of Fischer-Tropsch wastewater with the assistance of magnetite

Role of Methanosarcina in mercuric mercury transportation and methylation in sulfate-driven anaerobic oxidation of methane with municipal wastewater sludge

Sulfate-driven anaerobic oxidation of methane (AOM) and anaerobic digestion (AD) with municipal wastewater sludge containing heavy metals may provide favorable conditions for the biogeochemical transformation of mercury (Hg) by methanogens and methanotrophs. However, it remains largely unclear what Hg-methylators functioned and what role Methanosarcina played in these processes. Here, we performed sulfate-driven AOM following AD with Hg-containing wastewater sludge and investigated the role of microbes, especially Methanosarcina, in the biogeochemical transformation of Hg based on 16S rRNA amplicon and metatranscriptomic sequencing. Results showed that methylmercury (MeHg) concentrations and MeHg/total Hg ratios increased significantly, implying mercuric Hg [Hg(II)] methylation predominated MeHg demethylation. Desulfovibrio, Desulfobulbus and Methanosarcina dominated and thus likely played important roles in Hg(II) methylation, while Methanosarcina dominated and functioned in methane metabolism. In the presence of sulfate, differentially-expressed genes (DEGs) related to Hg transporting ATPase increased significantly, indicating Methanosarcina absorbed a large amount of Hg(II) and likely further methylated it to MeHg. No Hg response DEGs were found in the absence of sulfate, further confirming sulfate played an essential role in Hg cycle. Overall, these results suggest that controlling sulfate levels and Methanosarcina abundances in municipal wastewater could potentially mitigate MeHg risks to humans.

An in-depth analysis of microbial response to exposure to high concentrations of microplastics in anaerobic wastewater fermentation

The impact of microplastics (MPs) in anaerobic wastewater treatment on microbial metabolism is significant. Anaerobic granular sludge (AS) and biofilm (BF) are two common ways, and their responses to microplastics will have a direct impact on their application potential. This study investigated the microbial reactions of AS and BF to three types of MPs: polyethylene (PE), polyvinyl chloride (PVC), and a mixture of both (MIX). Results exhibited that MPs reduced methane output by 44.65 %, 55.89 %, and 53.18 %, elevated short-chain fatty acid (SCFA) levels by 95.93 %, 124.49 %, and 110.78 %, and lowered chemical oxygen demand (COD) removal by 28.77 %, 36.78 %, and 33.99 % for PE-MP, PVC-MP, and MIX-MP, respectively, with PVC-MP showing the greatest inhibition. Meanwhile, microplastics also facilitated the relative production of reactive oxygen species (ROS, 40.29 %-96.99 %), lactate dehydrogenase (LDH, 20.01 %-75.02 %), and adenosine triphosphate (ATP, 26.64 %-43.80 %), while reducing cytochrome c (cyt c, 23.60 %-49.02 %) and extracellular polymeric substances (EPS, 17.44 %-26.58 %). AS and BF displayed distinct enzymatic activities under MPs exposure. Correspondingly, 16S-rRNA sequencing indicated that AS was mainly involved in acetate generation by Firmicutes, while BF performed polysaccharide degradation by Bacteroidota. Metatranscriptomic analysis showed AS to be rich in acetogens (Bacillus, Syntrophobacter) and methanogens (Methanothrix, Methanobacterium), while BF contained more fermentation bacteria (Mesotoga, Lentimicrobium) and electroactive microorganisms (Clostridium, Desulfuromonas) under MIX-MP. Moreover, BF exhibited higher glycolysis gene expression, whereas AS was more active in methane metabolism, primarily through the acetoclastic methanogenic pathway's direct acetate conversion. This study provides new insights into understanding the microbial response produced by microplastics during anaerobic wastewater digestion.

Electric stimulation mitigated the mixed microplastic inhibition to anaerobic digestion during wastewater treatment

The presence of mixed microplastics (MPs) in anaerobic wastewater treatment processes has been shown to impede fermentation performance by suppressing microbial activity. Microbial electrosynthesis (MES), with its extensive potential, offers a promising solution for refractory substances management and methane recovery, achieved through the enhancement of microbial metabolism and interspecies electron transfer. This study, therefore, delves into the functional impacts and the microbial response to MES in the remediation of wastewater contaminated with mixed-MPs. Results indicated that mixed-MPs could inhibit methane production (-52.38%) and substance removal (-26.59%), and MES could effectively mitigate this inhibitory effect (-22.86%, -19.01%). Concurrently, MES also boosts enzymatic activities pivotal for electron transfer, such as cytochrome c and nicotinamide adenine dinucleotide (NADH), as well as those linked to energy metabolism like adenosine triphosphate (ATP). Furthermore, MES bolsters microbial resistance to mixed-MPs, as evidenced by an increase in extracellular polymeric substances (EPS), albeit with a minor rise in reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) release. Correspondingly, electric stimulation promoted the enrichment of functional microorganisms associated with fermentation, acetate production, electrogenesis, and methanogenesis, and stimulated elevated expression levels of genes related to methane metabolism. Notably, the Methanothrix-mediated acetoclastic pathway emerges as the predominant methanogenic route, succeeded by the Methanobacterium-driven hydrogenotrophic pathway. Lastly, the study underscores the supportive role of applied voltage and carriers in energy metabolism and substance transport, which are associated with methanogenesis. Overall, MES demonstrates efficacy in mitigating the biotoxicity induced by mixed-MPs exposure and in enhancing anaerobic wastewater treatment and methane recovery.

Optimization of sulfate reduction and methanogenesis via phase separation in a two-phase internal circulation reactor for the treatment of high-sulfate organic wastewater

To enhance the performance of the internal circulation (IC) reactor when treating high-sulfate organic wastewater, a laboratory-scale two-phase IC reactor with distinct phase separation capabilities was designed, and the sulfate reduction and methanogenesis processes were optimized by segregating the reactor into two specialized reaction zones. The results demonstrated that the first and second reaction areas of the two-phase IC reactor could be maintained at 4.5-6.0 and 7.5-8.5, respectively, turning them into the specialized phase for sulfate reduction and methanogenesis. Through phase separation, the two-phase IC reactor achieved a COD degradation and sulfate reduction efficiency of more than 80% when the influent sulfate concentration exceeded 5,000 mg/L, which were 32.32% and 16.04% higher than that before phase separation. Functional analyses indicated a greater activity of both the dissimilatory and assimilatory sulfate reduction pathways in the acidogenic phase, largely due to a rise in the relative abundance of the genera Desulfovibrio, Bacteroides, and Lacticaseibacillus, the primary carriers of sulfate reduction functional genes. In contrast, all the acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis pathways were inhibited in the acidogenic phase but thrived in the methanogenic phase, coinciding with shifts in the genus Methanothrix, which harbors the mcrA, mcrB, and mcrG genes essential for the final transformation step of all three methanogenesis pathways.

Bioelectrochemical anaerobic digestion mitigates microplastic pollution and promotes methane recovery of wastewater treatment in biofilm system

Bioelectrochemical systems (BES) offer significant potential for treating refractory waste and recovering bioenergy. However, their ability to mitigate microplastic pollution in wastewater remains unexplored. This study showed that BES facilitated the treatment of polyethylene (PE), polyvinyl chloride (PVC), and Mix (PE+PVC) microplastic wastewater and the methane recovery (40.61%, 20.02%, 21.19%, respectively). The lactate dehydrogenase (LDH), adenosine triphosphate (ATP), cytochrome c, and nicotinamide adenine dinucleotide (NADH/NAD+) ratios were elevated with electrical stimulation. Moreover, the applied voltage improved the polysaccharides content of the extracellular polymeric substances (EPS) in the PE-BES but decreased in PVC-BES, while the proteins showed the opposite trend. Metatranscriptomic sequencing showed that the abundance of fermentation bacteria, acetogens, electrogens, and methanogens was greatly enhanced by applying voltage, especially at the anode. Methane metabolism was dominated by the acetoclastic methanogenic pathway, with the applied voltage promoting the enrichment of Methanothrix, resulting in the direct conversion of acetate to acetyl-CoA via acetate-CoA ligase (EC: 6.2.1.1), and increased metabolic activity in the anode. Moreover, applied voltage greatly boosted the function genes expression level related to energy metabolism, tricarboxylic acid (TCA) cycle, electron transport, and transporters on the anode biofilm. Overall, these results demonstrate that BES can mitigate microplastic pollution during wastewater treatment.

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.

Quorum Sensing Enhances Direct Interspecies Electron Transfer in Anaerobic Methane Production

Direct interspecies electron transfer (DIET) provides an innovative way to achieve efficient methanogenesis, and this study proposes a new approach to upregulate the DIET pathway by enhancing quorum sensing (QS). Based on long-term reactor performance, QS enhancement achieved more vigorous methanogenesis with 98.7% COD removal efficiency. In the control system, methanogenesis failure occurred at the accumulated acetate of 7420 mg of COD/L and lowered pH of 6.04, and a much lower COD removal of 41.9% was observed. The more significant DIET in QS-enhancing system was supported by higher expression of conductive pili and the c-Cyts cytochrome secretion-related genes, resulting in 12.7- and 10.3-fold improvements. Moreover, QS enhancement also improved the energy production capability, with the increase of F-type and V/A-type ATPase expression by 6.3- and 4.2-fold, and this effect probably provided more energy for nanowires and c-Cyts cytochrome secretion. From the perspective of community structure, QS enhancement increased the abundance of Methanosaeta and Geobacter from 54.3 and 17.6% in the control to 63.0 and 33.8%, respectively. Furthermore, the expression of genes involved in carbon dioxide reduction and alcohol dehydrogenation increased by 0.6- and 7.1-fold, respectively. Taken together, this study indicates the positive effects of QS chemicals to stimulate DIET and advances the understanding of the DIET methanogenesis involved in environments such as anaerobic digesters and sediments.

Effect of zero-valent iron (ZVI) and biogas slurry reflux on methane production by anaerobic digestion of waste activated sludge

This study aimed to improve anaerobic digestion (AD) efficiency through the addition of zero-valent iron (ZVI) and biogas slurry. This paper demonstrated that methane production was most effectively promoted at a biogas slurry reflux ratio of 60%. The introduction of ZVI into anaerobic systems does not enhance its bioavailability. However, both biogas slurry reflux and the combination of ZVI with biogas slurry reflux increase the relative abundance of microorganisms involved in the direct interspecific electron transfer (DIET) process. Among them, the dominant microorganisms Methanosaeta, Methanobacterium, Methanobrevibacter, and Methanolinea accounted for over 60% of the total methanogenic archaea. The Tax4Fun function prediction results indicate that biogas slurry reflux and the combination of ZVI with biogas slurry reflux can increase the content of key enzymes in the acetotrophic and hydrotrophic methanogenesis pathways, thereby strengthening these pathways. The corrosion of ZVI promotes hydrogen production, and the biogas slurry reflux provided additional alkaline and anaerobic microorganisms for the anaerobic system. Their synergistic effect promoted the growth of hydrotrophic methanogens and improved the activities of various enzymes in the hydrolysis and acidification phases, enhanced the system's buffer capacity, and prevented secondary environmental pollution. PRACTITIONER POINTS: Optimal methane production was achieved at a biogas slurry reflux ratio of 60%. Biogas slurry reflux in anaerobic digestion substantially reduced discharge. ZVI addition in combination with biogas slurry reflux facilitates the DIET process.

Dual role of birnessite on the modulation of acid production and reinforcement of interspecific electron transfer in anaerobic digestion

Achieving efficient anaerobic digestion of highly loaded substrates is one of the most challenging problems in the field of waste resourcing. Here, the addition of birnessite (2.0 g/L) to kitchen wastewater increased the acetate and final methane yields by 40.53 and 99.18 %, respectively, while reducing the yields of propionate and butyrate by 38.17 and 48.86 %, respectively. There were two main pathways for birnessite to enhance anaerobic digestion, one of which is to act as an electron acceptor, by inducing an alteration in the ratio of reduced-state coenzyme I in the microorganism, allowing the acid production process to proceed towards deeper oxidation. Another pathway enhances the interspecific electron transfer between bacteria and archaea and improves methane yield by optimizing the metabolic relationship. The Kyoto Encyclopedia of Genes and Genomes (KEGG) functional predictions suggest that the extracellular electron transport pathway of the microorganism is enhanced with the addition of birnessite and that its intracellular metabolic pathway is biased towards the nicotinamide adenine dinucleotide (NADH) generation pathway. This work demonstrated that anaerobic digestion facilitation by metallic minerals was not monolithic; that is, different properties of the minerals were employed to intensify the different stages of anaerobic digestion and obtain an amplification cascade.

Metatranscriptome revealed how carbon brush addition affected the fermentation of food wastewater in the low-temperature environment

Improving the anaerobic digestion (AD) performance in low-temperature environments has become a key factor in the development of waste treatment and resource recovery in cold regions. The utilization of external carriers to form a biofilm is the simplest and most practical way to enhance the psychrophilic AD performance in cold regions. In this study, the effect of carrier addition on the fermentation performance of low-temperature (15 ± 2 °C) food wastewater was investigated by forming biofilms with carbon brushes. The results showed that although the biofilm formation enhanced methane yields (15.24%), it also caused more accumulation of propionic acid (306.99-626.89 mg/L), and the concentration of acetic acid (86.78-254.71 mg/L) was relatively low. The microbial community revealed the highest abundance of the fermentative bacterium Firmicutes and the carbon brush carrier significantly increased its relative abundance (23.74%). Metatranscriptomic sequencing revealed that the abundance level of Clostridium, Bacteroides, Sedimentibacter and Pelotomaculum was the highest, reaching 80% in all groups. In addition, the abundance level of electroactive microorganisms in biofilms was higher, while the fermentation bacteria and methanogens were lower. This showed that biofilm can enrich more electroactive microorganisms, and granular sludge needs to enrich more fermentation bacteria and methanogens to ensure metabolic activity. Further studies have found that carbon metabolism had the highest activity (27.86%-30.39%) and H+-transporting ATPase (atp) was the most dominant functional enzyme (85.50%-86.65%) involved in electron transport in low-temperature fermentation of food wastewater. Interestingly, these expression levels of active granular sludge were higher than the biofilm formed by carbon brushes. Meanwhile, analysis of the methanogenic pathway found that active granular sludge tends to be directly metabolized to realize acetate to acetyl-CoA by acetyl-CoA synthetase (ACSS), while biofilms were not significantly different in the two metabolic pathways of acetate. These results deepen the understanding of treating low-temperature food wastewater.

Analysis on methane production from various coal slime fermentations based on metagenomics

Metagenomic sequencing technology was applied to evaluate differences in the anaerobic fermentation process of coal slimes by analyzing microbial diversity, functional activity structure, and cooperative relationship during the anaerobic fermentation of coal slimes with different coal ranks. The obtained results showed that the production of biomethane from coal slime was decreased by increasing metamorphism degree. Internal reason was higher abundance of microbial community in low rank coal slimes compared to that in high rank coal which had higher activity in the gene expression of key steps such as hydrolysis and acidification, methanation and the production of hydrogen and acetic acid. Acetic acid decarboxylation and CO₂ reduction are two key pathways of methanation process. At the same time, K11261 (formylmethanofuran dehydrogenase subunit) and K01499 (methenyltetrahydromethanopterin cyclohydrolase) genes were further enriched in low rank slime systems, which enhanced the proportion of CO₂ reduction in methanation pathway and was beneficial to biomethane production. Research revealed the roles of different coal slime ranks in biomethane production process and is considered as an important reference significance for further exploration of coal slime resource utilization.

Internal driving mechanism of microbial community and metabolic pathway for psychrophilic anaerobic digestion by microbial electrolysis cell

The system that microbial electrolysis cell coupled anaerobic digestion (termed MEC-AD) with metal organic framework-modified cathode was operated under different voltage levels (0-1.2 V) at 20 °C. The maximum methane yield increased to 0.23 ± 0.01 LCH₄ g^-1COD at 0.9 V, with 28% improvement compared to 0 V (0.18 ± 0.01 LCH₄ g^-1COD). Moreover, total volatile fatty acid and propionate accumulation decreased by 32% and 15% at 0.9 V, indicating the system has potential to alleviate acidity suppression. Acidogens and electroactive microorganisms was clearly enriched with increasing applied voltage. Specifically, the abundance of Smithella increased, which could degrade propionate to acetate. Methanosaeta was dominant, accounting for ca. 40.1%∼55.1% of the archaea community at 0.3-1.2 V. Furthermore, the system reinforced psychrophilic methanogenesis by activating important enzymes involved in related metabolism pathways. Overall, this study provides perspective on the future practical application for the regulation of psychrophilic AD in electrochemically integrated bioreactors.

Dissimilatory manganese reduction facilitates synergistic cooperation of hydrolysis, acidogenesis, acetogenesis and methanogenesis via promoting microbial interaction during anaerobic digestion of waste activated sludge

Anaerobic digestion (AD) of waste activated sludge (WAS) is commonly limited to poor synergistic cooperation of four stages including hydrolysis, acidogenesis, acetogenesis and methanogenesis. Dissimilatory metal reduction that induced by metal-based conductive materials is promising strategy to regulate anaerobic metabolism with the higher metabolic driving force. In this study, MnO₂ as inducer of dissimilatory manganese reduction (DMnR) was added into WAS-feeding AD system for mediating complicated anaerobic metabolism. The results demonstrated that main operational performances including volatile solid (VS) degradation efficiency and cumulative CH₄ production with MnO₂ dosage of 60 mg/g·VS reached up to maximum 53.6 ± 3.4% and 248.2 ± 10.1 mL/g·VS while the lowest operational performances in control group (38.5 ± 2.8% and 183.5 ± 8.5 mL/g·VS) was originated from abnormal operation of four stages. Furthermore, high-throughput 16 S rRNA pyrosequencing revealed that enrichment of dissimilatory manganese-reducing contributors and methanogens such as Thermovirga, Christensenellaceae_R_7_group and Methanosaeta performed the crucial role in short-chain fatty acids (SCFAs) oxidation and final methanogenesis, which greatly optimized operational environment of hydrolysis, acidogenesis and acetogenesis. More importantly, analysis of functional genes expression proved that abundances of genes encoding enzymes participated in acetate oxidation, direct interspecies electron transfer (DIET) and CO₂ reduction pathway were simultaneously up-regulated with the optimum MnO₂ dosage, suggesting that DMnR with SCFAs oxidation as electron sink could benefit stable operation of four stages via triggering effective DIET-based microbial interaction mode.

Enhancing methane production of co-digested food waste with granular activated carbon coated with nano zero-valent iron in an anaerobic digester

Anaerobic digestion (AD) possesses dual benefits of waste treatment and energy generation. The use of conductive additives in AD matrix has potential to improve process yield. Hence, the study aimed to investigate a thermophilic AD (TAD) inserted by granular activated carbon coated with nano zero-valent iron (GAC/nZVI) in the matrix and was operated for mono-digestion and co-digestion of cow manure with food wastes (rice and bread) to check the bioprocess improvement. The results were compared with the control TAD without conductive additives. Biogas production increased by 11 folds upon using GAC/nZVI addition compared to the control TAD. Moreover, the addition of GAC/nZVI increased the methane in biogas by 20.7 folds compared to control one. With GAC/nZVI, the maximum COD removal of 78.29% and 85.21% were noticed for co-digestion and mono digestion, respectively. Such improvement of TAD performance was due to easy bacterial communication and electron exchange through the conductive particles.

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.

Iron Compounds in Anaerobic Degradation of Petroleum Hydrocarbons: A Review

Waste and wastewater containing hydrocarbons are produced worldwide by various oil-based industries, whose activities also contribute to the occurrence of oil spills throughout the globe, causing severe environmental contamination. Anaerobic microorganisms with the ability to biodegrade petroleum hydrocarbons are important in the treatment of contaminated matrices, both in situ in deep subsurfaces, or ex situ in bioreactors. In the latter, part of the energetic value of these compounds can be recovered in the form of biogas. Anaerobic degradation of petroleum hydrocarbons can be improved by various iron compounds, but different iron species exert distinct effects. For example, Fe(III) can be used as an electron acceptor in microbial hydrocarbon degradation, zero-valent iron can donate electrons for enhanced methanogenesis, and conductive iron oxides may facilitate electron transfers in methanogenic processes. Iron compounds can also act as hydrocarbon adsorbents, or be involved in secondary abiotic reactions, overall promoting hydrocarbon biodegradation. These multiple roles of iron are comprehensively reviewed in this paper and linked to key functional microorganisms involved in these processes, to the underlying mechanisms, and to the main influential factors. Recent research progress, future perspectives, and remaining challenges on the application of iron-assisted anaerobic hydrocarbon degradation are highlighted.

Enhanced carbon dioxide biomethanation with hydrogen using anaerobic granular sludge and metal-organic frameworks: Microbial community response and energy metabolism analysis

In this work, metal-organic frameworks (MOFs) were prepared to evaluate its impact on carbon dioxide (CO₂) biomethanization during anaerobic degradation (AD). The results showed that MOFs significantly improved the CO₂ biomethanation efficiency, especially in the AD reactors using a concentration of 1.0 g/L MOFs. Furthermore, MOFs promoted direct interspecific electron transfer and alleviated the hydrogen competition of bacteria. Meanwhile, hydrogenotrophic methanogens were enriched in the AD reactors with MOFs. After the addition of MOFs, there was 3.28 times and 3.41 times increase in the abundance of metabolic functions related to methanogenesis by CO₂ reduction with hydrogen and dark hydrogen oxidation, respectively. There was an increased abundance of all genes that encode the key enzymes used in methane metabolism. However, functional genes involved in nitrate reduction had their expressions inhibited. The work may offer a contribution to helping the industry achieve the carbon capture and utilization policy.

Utilization of Response Surface Methodology in Optimization and Modelling of a Microbial Electrolysis Cell for Wastewater Treatment Using Box–Behnken Design Method

A vast quantity of untreated wastewater is discharged into the environment, resulting in contamination of receiving waters. A microbial electrolysis cell (MEC) is a promising bioelectrochemical system (BES) for wastewater treatment and energy production. However, poor design and control of MEC variables may lead to inhibition in the system. This study explored the utilization of Response Surface Methodology (RSM) on the synergistic aspects of MEC and magnetite nanoparticles for wastewater treatment. Influences of temperature (25–35 °C), voltage supply (0.3–1.3 V) and magnetite nanoparticle dosage (0.1–1.0 g) on the biochemical methane potentials (BMPs) were investigated with the aim of optimizing biogas yield, chemical oxygen demand removal and current density. The analysis of variance (ANOVA) technique verified that the quadratic models obtained were substantial, with p-values below 0.05 and high regression coefficients (R2). The optimum biogas yield of 563.02 mL/g VSfed, chemical oxygen demand (COD) removal of 97.52%, and current density of 26.05 mA/m2 were obtained at 32.2 °C, 0.77 V and 0.53 g. The RSM revealed a good comparison between the predicted and actual responses. This study revealed the effective utilization of statistical modeling and optimization to improve the performance of the MEC to achieve a sustainable and eco-friendly situation.

Potential of biogas residue biochar modified by ferric chloride for the enhancement of anaerobic digestion of food waste

Biogas residue biochar (BRB) and BRB modified by ferric chloride (BRB-FeCl₃) were applied to promote anaerobic digestion (AD) of food waste (FW), related mechanisms were also proposed in this study. Results indicated BRB-FeCl₃ showed higher specific surface area, more abundant functional groups and impregnate iron than BRB, and they respectively increased 22.50% and 12.79% cumulative methane yields compared with control group because of accelerated volatile fatty acids (VFAs) transformation, which were confirmed by enhanced metabolism of glycolysis, fatty acid degradation and pyruvate. BRB, especially BRB-FeCl₃ facilitated the growth of Syntrophomonas, Methanofollis, Methanoculleus and Methanosarcina, which further promoted the methanogenesis by enhancing the metabolic activities of methanol, dimethylamine and methylamine pathways, thereby causing more metabolically diverse methanogenic pathways. Metagenomics analysis revealed BRB, especially BRB-FeCl₃ promoted the relative abundances of functional genes involved in direct interspecies electron transfer (DIET). Present study explored the enhancement mechanisms and feasibility of BRB-FeCl₃ for AD process.

Strategy to enhance semi-continuous anaerobic digestion of food waste by combined use of calcium peroxide and magnetite

Optimizing methane production from food waste (FW) efficiently is always a hot topic in the field of anaerobic digestion (AD). In this study we aimed to improve the conversion of organics to methane by using CaO₂ and magnetite to enhance the semi-continuous AD of food waste. Under the organic load of 2.5 g VS/L·d^-1, the specific methane yield was increased from 333.9 mL CH₄/g·VS to 423.4 mL CH₄/g·VS by adding 0.01 g/L CaO₂ with 0.4 g/L magnetite, improving the production of methane from FW. We assessed reactor performance, ORP changes, mass balance, enzyme activities and characterized the metagenomic profile of microorganisms involved in digestion. These microorganisms showed rapid conversion of volatile fatty acids and increased expression of genes related to hydrolysis and acid production. Thus, the addition of CaO₂ and magnetite optimized the relationship between fermentation bacteria and methanogenic archaea to enhance the overall production of methane. Microorganisms evolved unique adaptive mechanisms in the co-operative environment of CaO₂ and magnetite, as their energy metabolism patterns combined those controlled by individual CaO₂ and magnetite addition. This method of combining a micro-aeration environment with conductive materials provides a new perspective for optimizing the AD of FW.

A comprehensive review on the use of conductive materials to improve anaerobic digestion: Focusing on landfill leachate treatment

Globally, around 70% of waste is disposed of in open dumps or landfill sites, with the leachate generated from these sites containing high concentrations of organic and inorganic compounds, which will adversely affect aquatic environments if discharged without proper treatment. Anaerobic digestion of landfill leachate is an environmentally-friendly method that efficiently converts organic compounds into methane-rich biogas. However, the widespread application of anaerobic digestion has been hindered by poor system stability, low methanogenic activity and a high level of volatile fatty acids (VFAs) accumulation, increasing the operational costs of treatment. Conductive materials can be added to the digester to improve the performance of anaerobic digestion in landfill leachate treatment systems and studies reporting the use of conductive materials for this purpose are hereby thoroughly reviewed. The mechanism of microbial growth and enrichment by conductive materials is discussed, as well as the subsequent effect on waste metabolism, methane production, syntrophic relationships and interspecies electron transfer. The porous structure, specific surface area and conductivity of conductive materials play vital roles in the facilitation of syntrophic relationships between fermentative bacteria and methanogenic archaea. In addition, the mediation of direct interspecies electron transfer (DIET) by conductive materials increases the methane content of biogas from 16% to 60% as compared to indirect interspecies electron transfer (IIET) in conventional anaerobic digestion systems. This review identifies research gaps in the field of material-amended anaerobic systems, suggesting future research directions including investigations into combined chemical-biological treatments for landfill leachate, microbial management using conductive materials for efficient pollutant removal and the capacity for material reuse. Moreover, findings of this review provide a reference for the efficient and large-scale treatment of landfill leachate by anaerobic digestion with conductive materials.

Effects of Iron Powder Addition and Thermal Hydrolysis on Methane Production and the Archaeal Community during the Anaerobic Digestion of Sludge

The conventional anaerobic digestion of sludge has the disadvantages of long digestion time and low methane production. Pretreatment is often used to mitigate these problems. In this study, three pretreatment methods, namely, the addition of iron powder, high-temperature thermal hydrolysis, and a combination of these methods, were compared for application with conventional continuous anaerobic digestion reactors. The results showed that pretreatment improved methane yield by 18.2–22.9%, compared to the control reactor (conventional anaerobic digestion). Moreover, it was recognized that the archaeal community in the sludge underwent significant changes after pretreatment. Specifically, the addition of iron powder reduced the diversity in the archaeal community, but increased the abundance of hydrogenotrophic methanogens without changing the community composition. Thermal hydrolysis at high temperatures had the reverse effect, as it increased the diversity of the archaeal community but inhibited the growth of acetoclastic methanogens. In the case of the combined pretreatment, the thermal hydrolysis had a dominant influence on the archaeal community. By comparing the changes in functional gene content, it was found that the functional abundance of the archaeal community in the transport and metabolism of carbohydrates, lipids, and amino acids was higher after pretreatment than in the control group.

Use of bag-filter gas dust in anaerobic digestion of cattle manure for boosting the methane yield and digestate utilization

Plenty of refractory and environmentally hazardous bag-filter gas dust (BGD) is produced in the iron-making process. The effects of untreated BGD on anaerobic digestion (AD) with cattle manure were investigated. The BGD had the potential to boost the methane yield and digestate utilization considerably. The digester with 2.0 wt% BGD gained the highest methane yield (256.3 mL/g VS) and chemical oxygen demand removal rate (56.8%), 51.3% and 20.1% higher than that (169.4 mL/g VS, 47.3%) of the control group, respectively. The digestates with BGD possessed a utilization potential with the stability of 49.5-57.9% and fertility of 4.65-4.86%. Electrochemical measurements demonstrated that 2.0 wt% BGD improved the electron transport capacity of the AD system and increased the limiting current and redox peak current by 40.3% and 12.9%, respectively. A strategy for understanding the BGD reinforcing methanogenesis was proposed. This work also provides a technical support for recycling the BGD.

A collaborative strategy for enhanced anaerobic co-digestion of food waste and waste activated sludge by using zero valent iron and ferrous sulfide

The application of sulfidated zero-valent iron as an alternative used in coupled anaerobic systems to improve methane production is usually restricted by its high production costs and toxic gasses and wastewater generation. In this study, a collaborative strategy for coupling zero-valent iron (ZVI) and ferrous sulfide (FeS) together into anaerobic systems was used to evaluate the enhancement of methanogenesis during the co-digestion of food waste and waste activated sludge, with the microbial evolution and metabolic pathway revealed. Results showed that the enhanced hydrolysis and acidogenesis process of co-digestion in this coupled anaerobic system could be attributed to synergistic interactions among ZVI, FeS, and microorganisms. Furthermore, both acetoclastic and hydrogenotrophic pathways could be promoted by coupling ZVI and FeS. This study demonstrated that coupling ZVI and FeS together into anaerobic systems would be a promising method for improving the methanogenic performance for municipal solid waste treatment.

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.

Promotion of methane production by magnetite via increasing acetogenesis revealed by metagenome-assembled genomes

Metal oxides are wildly studied to enhance anaerobic digestion and the methanogenic process, which is generally interpreted by increased direct interspecies electron transfer (DIET). Yet microbial mechanisms involved are under debate. Herein, methane production dynamics were analyzed, and acceleration on biogas accumulation was presented. Complementing previous findings, Fe₃O₄ nanoparticles stimulated bacterial fermentation rather than methanogenesis or syntropy between electro-microorganism and methanogen. More importantly, metagenome-assembled genomes proved that Fe₃O₄ nanoparticles increased acetogenesis by Parabacteroides chartae, which provided abundant substrates for acetoclastic methanogenesis. Interestingly, the weakly conductive V₃O₇·H₂O nanowires increased potential hydrogen-producing bacteria, Brevundimonas, and electro-microorganisms, Clostridium and Rhodoferax, which is convenient for conducting DIET. Collectively, conductivity may not be a critical factor in mediating DIET and distinct strategies of metal oxides on methane production propose more possibilities, such as fermentation process.

Biofilm formation as a method of improved treatment during anaerobic digestion of organic matter for biogas recovery

The efficiency of anaerobic digestion could be increased by promoting microbial retention through biofilm development. The inclusion of certain types of biofilm carriers has differentiated existing AD biofilm reactors through their respective mode of biofilm growth. Bacteria and archaea engaged in methanogenesis during anaerobic processes potentially build biofilms by adhering or attaching to biofilm carriers. Meta-analyzed results depicted varying degrees of biogas enhancement within AD biofilm reactors. Furthermore, different carrier materials highly induced the dynamicity of the dominant microbial population in each system. It is suggested that the promotion of surface contact and improvement of interspecies electron transport have greatly impacted the treatment results. Modern spectroscopy techniques have been and will continue to give essential information regarding biofilm's composition and structural organization which can be useful in elucidating the added function of this special layer of microbial cells.

The biochemical mechanism of enhancing the conversion of chicken manure to biogenic methane using coal slime as additive

To improve the efficiency of methane production from chicken manure (CM) anaerobic digestion, the mechanism of coal slime (CS) as an additive on methane production characteristics were investigated. The results showed that adding an appropriate amount of CS quickened the start of the fermentation and effectively increased the methane yield. In addition, the pH changed in a stable manner in the liquid phase, and the concentrations of total ammonia nitrogen (TAN) and free ammonia nitrogen (FAN) were reduced. Moreover, organic matter was decomposed and volatile fatty acids (VFAs) were consumed effectively. The abundance of Bacteroides in the bacterial community and Methanosarcina in the archaea was increased. In addition, the reduction of CO₂ was the main methanogenic pathway, and adding CS raised the abundance of genes for key enzymes in metabolic pathways during methane metabolism. The results provide a novel method for the efficient methane production from CM.

Promotion of direct interspecies electron transfer and potential impact of conductive materials in anaerobic digestion and its downstream processing - a critical review

Addition of conductive materials (CMs) has been reported to facilitate direct interspecies electron transfer (DIET) and improved anaerobic digestion (AD) performance. This review summarises the benefits and outlines remaining research challenges of the addition of CMs with a focus on the downstream processing of AD. CM addition may alter biogas quality, digestate dewaterability, biosolids volume, and centrate quality. Better biogas quality has been observed due to the adsorption of H₂S to metallic CMs. The addition of CMs results in an increase in solid content of the digestate and thus an additional requirement for sludge dewatering and handling and the final biosolids volume for disposal. This review highlights the need for more research at pilot scale to validate the benefits of CM addition and to evaluate CM selection, doses, material costs, and the impact on downstream processes. The lack of research on the impact of CMs on the downstream process of AD is highlighted.

The metabolic process of methane production by combined fermentation of coal and corn straw

The anaerobic degradation of coal combined with straw biomass can promote the methane production. The biogas production potential and metabolic pathway were explored via the co-digestion simulation experiment of coal and corn straw. The results showed that 2 g of corn straw combined respectively with 4 g of bituminous coal A, 6 g of bituminous coal B and 4 g of bituminous coal C resulted in highest methane yields. The structure of lignocellulose in corn straw was partially degraded into guaiacyl and syringyl units. Meanwhile, the content of biodegradable tyrosine like protein and soluble microbial by-products in liquid phase significantly decreased. Significantly, the structure of archaea altered from aceticlastic to hydrogenotrophic methanogens when the fermentation substrate changed from high to low rank coal. The proportion of hydrogenotrophic methanogens was significantly higher than that of aceticlastic and methylotrophic methanogens, and the hydrogenotrophic pathway was dominant than the aceticlastic pathway.

Dual-chamber differs from single-chamber microbial electrosynthesis in biogas production performance under low temperature (15℃)

In this study, single-chamber and dual-chamber Microbial electrosynthesis (MES) with carbon fiber brushes as electrodes were operated at 15°C to compare and analyze the difference in methanogenic performance. Metatranscriptomic analysis showed that the relative abundance of electroactive microorganisms Syntrophomonas, Pseudomonas and Bacteroides in each group exceeded 90%, while the abundance of Geobacter was less than 4%. Acetoclastic methanogens Methahnosarcina was more enriched in dual-chamber MES (61.74%~70.42%), and Methanothrix showed higher abundance in single-chamber MES (33.44%~51.71%). Methahnosarcina and Methanothrix could interact with electroactive microorganisms to improve the electron transfer efficiency through direct interspecies electron transfer (DIET). Analysis of the methane metabolic pathways of low-temperature MES found acetoclastic pathway was domination, and single-chamber MES achieved acetate to acetyl-CoA through acetate-CoA ligase (EC: 6.2.1.1), whereas dual-chamber MES was by acetate kinase (EC: 2.7.2.1) and phosphate acetyltransferase (EC: 2.3.1.8). These results are beneficial to further research on the treatment of low-temperature wastewater.

Simultaneously autotrophic denitrification and organics degradation in low-strength coal gasification wastewater (LSCGW) treatment via microelectrolysis-triggered Fe(II)/Fe(III) cycle

The autotrophic iron-depended denitrification (AIDD), triggered by microelectrolysis, was established in the microelectrolysis-assistant up-flow anaerobic sludge blanket (MEA-UASB) with the purpose of low-strength coal gasification wastewater (LSCGW) treatment while control UASB operated in parallel. The results revealed that chemical oxygen demand (COD) removal efficiency and total nitrogen (TN) removal load at optimum current (2.5 A/m³) in MEA-UASB (83.2 ± 2.6% and 0.220 ± 0.010 kg N/m³·d) were 1.42-fold and 1.57-fold higher than those (58.5 ± 2.1% and 0.139 ± 0.011 kg N/m³·d) in UASB, verifying that AIDD and following dissimilatory iron reduction (DIR) process could offer the novel pathway to solve the electron donor-deficient and traditionally denitrification-infeasible problems. High-throughput 16S rRNA gene pyrosequencing shown that iron-oxidizing denitrifiers (Thiobacillus and Acidovorax species) and iron reducing bacteria (Geothrix and Ignavibacterium speices), acted as microbial iron cycle of contributors, were specially enriched at optimum operating condition. Additionally, the activities of microbial electron transfer chain, electron transporters (complex I, II, III and cytochrome c) and abundance of genes encoding important enzymes (narG, nirK/S, norB and nosZ) were remarkably promoted, suggesting that electron transport and consumption capacities were stimulated during denitrification process. This study could shed light on better understanding about microelectrolysis-triggered AIDD for treatment of refractory LSCGW and further widen its application potential in the future.

Enhancing anaerobic digestion process with addition of conductive materials

Anaerobic digestion is widely used for the treatment of wastewater for its low costs and bioenergy production, but the performances of anaerobic digestion often need improving in practical applications. The addition of conductive materials could lead to direct interspecies electron transfer (DIET) among the anaerobic microorganisms, and consequently enhance the efficiencies of anaerobic digestion. In this paper, the effects of DIET via conductive materials on chemical organic demand (COD) removal, volatile fatty acid (VFA) consumption and methane production were reviewed. The reports on the increase of conductive microorganisms due to the addition of conductive materials were discussed. Results regarding activities of microorganisms and morphology and properties of sludge were described and commented, and future research needs were also proposed which included better understanding of the roles of DIET in each step of anaerobic digestion, mechanisms of metabolism of pollutants in DIET-established systems and inhibition of excessive dosage of conductive materials.

Biochar enhanced high-solid mesophilic anaerobic digestion of food waste: Cell viability and methanogenic pathways

The underlying mechanisms of biochar enhance high-solid anaerobic digestion (HSAD) of food waste were investigated with a focus on the cell viability, microbial community, and methanogenic pathways. This study assessed the effects of different dosages of biochar in HSAD. Optimal biochar dosage was found to be 25 g/L, which produced accumulative methane yields of up to 251 mL CH₄/g VS significantly promote volatile fatty acid degradations, especially in butyric acid concentrations. Effects of biochar with a dosage of 25 g/L on the cell viability showed that viable cells based on cell membrane integrity increased from 2.9% to 6.4%. Meanwhile, intact and highly active cells with high DNA content were probably involved in direct interspecies electron transfer (DIET) via membrane-bound electron transport proteins. Further analysis demonstrated that Syntrophomonas and methanogens Methanosarcina &Methanocelleus were selectively enriched by biochar, which resulted in the methanogenic pathways shifting from acetoclastic/hydrogenotrophic methanogenic pathways to more metabolically diverse methanogenic pathways. Accordingly, biochar-mediated DIET was possibly established between Syntrophomonas and Methanosarcina species due to those viable cells. In conclusion, biochar is a feasible additive in enhancing HSAD methanogenic performance.

Link between characteristics of Fe(III) oxides and critical role in enhancing anaerobic methanogenic degradation of complex organic compounds

Fe(III) oxides have been investigated to accelerate anaerobic methanogenic degradation of complex organic compounds. However, the critical role linked to the characteristics of different types of Fe(III) oxides is still unclear. Study presented here performed a side-by-side comparison of four types of Fe(III) oxides including Fe(III)-citrate, ferrihydrite, hematite and magnetite to evaluate their effectiveness in methanogenic degradation of phenol. Results showed that, amorphous Fe(III)-citrate group showed the fastest phenol degradation and Fe²⁺ release among all the groups, followed by poorly crystalline ferrihydrite. Although Fe(III)-citrate group also showed the fastest methane production rate, the efficiency of electron recovery in methane production was only 58-78%, which was evidently lower than that in both crystalline hematite (86-89%) and magnetite (93-97%) groups. Methane production rate with non-conductive ferrihydrite was nearly same as that with conductive magnetite, both of which were significantly higher than that with semi-conductive hematite. X-ray Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis showed that sludge collected from hematite and magnetite group still respectively presented a relatively intact characteristic spectra involved in hematite and magnetite. Differently, the characteristic spectra involved in ferrihydrite was not evident in sludge collected from ferrihydrite group, whereas the characteristic spectra involved in magnetite was detected. Microbial community analysis showed that, both Fe(III)-citrate and ferrihydrite specially enriched Fe(III)-reducing bacteria capable of degrading phenol into fatty acids (Trichococcus and Caloramator) via dissimilatory Fe(III) reduction. Fe(III)-citrate also stimulated the growth of Syntrophus capable of degrading phenol/benzoate into acetate and proceeding direct interspecies electron transfer (DIET). In magnetite and hematite group, the abundance of Enterococcus species evidently increased, and they might proceed DIET with Methanothrix species in syntrophic conversion of fatty acids into methane.

Role of MnO2 in controlling iron and arsenic mobilization from illuminated flooded arsenic-enriched soils

This study examined the role of intermittent illumination/dark conditions coupled with MnO₂-ammendments to regulate the mobility of As and Fe in flooded arsenic-enriched soils. Addition of MnO₂ particles with intermittent illumination led to a pronounced increase in the reductive-dissolution of Fe(III) and As(V) from flooded soils compared to a corresponding dark treatments. A higher MnO₂ dosage (0.10 vs 0.02 g) demonstrated a greater effect. Over a 49-day incubation, maximum Fe concentrations mobilized from the flooded soils amended with 0.10 and 0.02 g MnO₂ particles were 2.39 and 1.85-fold higher than for non-amended soils under dark conditions. The corresponding maximum amounts of mobilized As were at least 92 % and 65 % higher than for non-amended soils under dark conditions, respectively. Scavenging of excited holes by soil humic/fulvic compounds increased mineral photoelectron production and boosted Fe(III)/As(V) reduction in MnO₂-amended, illuminated soils. Additionally, MnO₂ amendments shifted soil microbial community structure by enriching metal-reducing bacteria (e.g., Anaeromyxobacter, Bacillus and Geobacter) and increasing c-type cytochrome production. This microbial diversity response to MnO₂ amendment facilitated direct contact extracellular electron transfer processes, which further enhanced Fe/As reduction. Subsequently, the mobility of released Fe(II) and As(III) was partially attenuated by adsorption, oxidation, complexation and/or coprecipitation on active sites generated on MnO₂ surfaces during MnO₂ dissolution. These results illustrated the impact of a semiconducting MnO₂ mineral in regulating the biogeochemical cycles of As/Fe in soil and demonstrated the potential for MnO₂-based bioremediation strategies for arsenic-polluted soils.

New insights into enhanced anaerobic degradation of coal gasification wastewater (CGW) with the assistance of magnetite nanoparticles

Magnetite nanoparticles (Fe₃O₄ NPs) was firstly used to enhance pollutants removal during coal gasification wastewater (CGW) treatment in anaerobic digestion (AD) system. Bench-scale results revealed that 200 mg/L and 20-40 nm of Fe₃O₄ NPs addition resulted in a maximum removal capacity of total phenol (TPh) at a temperature of 36 °C and hydraulic retention time (HRT) of 36 h. Meanwhile, Fe₃O₄ NPs addition reduced the oxidation reduction potential (ORP) values and biological toxicity, and enhanced the stability of AD system. Pilot-scale results showed that the TPh and chemical oxygen demand (COD) removal efficiency (53% and 49%) were obtained with the optimal dosage of Fe₃O₄ NPs. Moreover, electron nanowires may be established with Fe₃O₄ NPs assisted to perform direct interspecies electron transfer (DIET) among Geobacter, Pseudomonas and Methanosaeta species, and finally enhanced the pollutants removal efficiency.

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.

Metagenomic insights into aniline effects on microbial community and biological sulfate reduction pathways during anaerobic treatment of high-sulfate wastewater

For comprehensive insights into the change of sulfate reduction pathway responding to the toxic stress and the shift of microbial community and performance of sulfate reduction, we built a laboratory-scale expanded granular sludge bed reactor (EGSB) treating high-sulfate wastewater with elevated aniline concentrations from 0 to 480 mg/L. High-throughput sequencing and metagenomic approaches were applied to decipher the molecular mechanisms of sulfate reduction under aniline stress through taxonomic and functional profiles. The increasing aniline in the anaerobic system induced the accumulation of volatile fatty acids (VFA), further turned the bioreactor into acidification, which was the principal reason for the deterioration of system performance and finally resulted in the accumulation of toxic free sulfide. Moreover, aniline triggered the change of bacterial community and genes relating to sulfate reduction pathways. The increase of aniline from 0 to 320 mg/L enriched total sulfate-reducing bacteria (SRB), and the most abundant genus was Desulfomicrobium, accounting for 66.85-91.25% of total SRB. The assimilatory sulfate reduction pathway was obviously inhibited when aniline was over 160 mg/L, while genes associated with dissimilatory sulfate reduction pathways all exhibited an upward tendency with the increasing aniline content. The enrichment of aniline-resistant SRB (e.g. Desulfomicrobium) carrying genes associated with the dissimilatory sulfate reduction pathway also confirmed the underlying mechanism that sulfate reduction turned into dissimilation under high aniline condition. Taken together, these results comprehensively provided solid evidence for the effects of aniline on the biological sulfate reduction processes treating high-sulfate wastewater and the underlying molecular mechanisms which may highlight the important roles of SRB and related sulfate reduction genes during treatment.

A two-stage desalination process for zero liquid discharge of flue gas desulfurization wastewater by chloride precipitation

A two-stage desalination process was developed to achieve zero liquid discharge (ZLD) of flue gas desulfurization (FGD) wastewater by precipitating chloride as Friedel's salt. Influential factors for Friedel's salt precipitation, including dosage, reaction time, concentration of sulfate, were investigate by batch tests. Batch results showed that at calcium to aluminum molar ratio of 3.0, the optimal chloride removal and the highest crystallinity of Friedel's salt were obtained. Sulfate impeded Friedel's salt precipitation by competitive inhibition mechanism, and thus calcium sulfate removal was designed in advance of chloride removal. Batch results and long-term results of bench-scale experiments showed that magnesium and part of sulfate were effectively removed by lime addition in Stage I of the proposed process, and then the remaining sulfate and 48.1 % of chloride were precipitated as ettringite and Friedel's salt in Stage II. The effluent of the two-stage process was alkaline with low turbidity, and had considerable desulfurization capacity. Techno-economic evaluation showed that the two-stage process is technically feasible, economically viable and environmentally friendly technology for ZLD of FGD wastewater.

Performance, mechanism and stability of nitrogen-doped sewage sludge based activated carbon supported magnetite in anaerobic degradation of coal gasification wastewater

In current study, the UASB reactor was enhanced by nitrogen-doped sewage sludge based activated carbon supported Fe₃O₄ (Fe₃O₄/N-SBAC) for coal gasification wastewater treatment. The results showed that COD removal efficiency was increased to 64.4% with Fe₃O₄/N-SBAC assistance and the corresponding methane production rate achieved up to 1093.6 mL/d. Fe₃O₄/N-SBAC promoted microbial growth and enzymatic activity, leading to high extracellular polymeric substances and coenzyme F₄₂₀ concentrations. Fe₃O₄/N-SBAC also facilitated the sludge granulation process with high particle size, substantial interspecific signal molecules and low diffusible signal factor. Microbial community analysis revealed that Fe₃O₄/N-SBAC might support direct interspecies electron transfer process, in which the enriched Geobacter was likely to communicate with Methanothrix via electrical connection, improving anaerobic degradation of coal gasification wastewater. Total phenols shock and pH impact revealed that reactor stability was enhanced in the Fe₃O₄/N-SBAC-supplemented system.

Ecological and functional research into microbiomes for targeted phenolic removal in anoxic carbon-based fluidized bed reactor (CBFBR) treating coal pyrolysis wastewater (CPW)

Powdered activated carbon (PAC), lignite activated coke (LAC) and Fe-C carriers were applied to enhance CBFBRs to degrade targeted phenolics. In start-up stage, PAC and LAC equipped CBFBRs with higher environment adaptability and phenolic degradation capacity for phenol (>96%), p-cresol (>91%) and 3, 5-dimethylphenol (>84%) in comparison to Fe-C carrier. In recovery stage, the superior performance was also identified for CBFBRs in basis of PAC and LAC than Fe-C-based reactor. However, the Fe-C carrier assisted CBFBR with more stable degradation performance under impact loading. By comparing microbiomes, significantly enriched Brachymonas (54.80%-68.81%) in CBFBRs exerted primary role for phenolic degradation, and positively contributed to microbial network. Meanwhile, Geobacter in Fe-C-based reactor induced excellent impact resistance by enhancing interspecific electron transfer among microbes. Furthermore, the investigation on functional genes related to phenolic degradation revealed that anaerobic pathway accounted for demethylation procedure, while aerobic pathways dominated the phenolic ring-cleavage process.

Clarifying the synergetic effect of magnetite nanoparticles in the methane production process

Magnetite nanoparticles (Fe₃O₄ NPs) were applied in an anaerobic semi-continuous tank reactor (ASTR) to investigate its effect on the anaerobic digestion (AD) of acetate synthetic wastewater. The Fe₃O₄ NPs corrosion could create a more favorable micro-environment to enhance the methanogens activity. The chemical oxygen demand (COD) removal efficiency and methane production in test (ASTR_T) were 31.1% and 101.5% higher than those in control (ASTR_C). With the addition of Fe₃O₄ NPs, the concentration of key coenzyme (F₄₂₀ and M) increased from averagely 0.523 and 5.352 μmol/g-VSS to 0.956 and 9.267 μmol/g-VSS, and the content of soluble microbial products (SMPs) significantly increased. Additionally, the high-throughput 16S rRNA gene sequencing further confirmed that the percentage of hydrogen-utilizing methanogens (Methanolinea) was up to 62.6% of total archaeal sequences. Fe₃O₄ NPs addition would accelerate electrons transfer from acids oxidizers to syntrophic methanogenesis, further stimulate acids oxidizers to decompose acetate to H₂/CO₂, and finally facilitate more methane production.

Magnetite Synthesis in the Presence of Cyanide or Thiocyanate under Prebiotic Chemistry Conditions

Magnetite is an iron oxide mineral component of primitive Earth. It is naturally synthesized in different ways, such as magma cooling as well as olivine decomposition under hydrothermal conditions. It is probable magnetite played a significant role in biogenesis. The seawater used in the current work contained high Mg2+, Ca2+ and SO42- concentrations, unlike the seawater of today that has high Na+ and Cl- concentrations. It is likely that this seawater better resembled the ion composition of the seas of the Earth from 4 billion years ago. Cyanide and thiocyanate were common molecules in prebiotic Earth, and especially in primitive oceans, where they could act on the magnetite mechanism synthesis via Fe2+ interaction. In this research, magnetite samples that were synthesized under prebiotic conditions in the presence of cyanide or thiocyanate, (both with and without artificial seawater), showed that, besides magnetite, goethite and ferrihydrite can be produced through different Fe2+-ion interactions. Cyanide apparently acts as a protective agent for magnetite production; however, thiocyanate and seawater 4.0 Gy ions produced goethite and ferrihydrite at different ratios. These results validate that Fe3+ oxides/hydroxides were possibly present in primitive Earth, even under anoxic conditions or in the absence of UV radiation. In addition, the results show that the composition of water in early oceans should not be neglected in prebiotic chemistry experiments, since this composition directly influences mineral formation.

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

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

Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe₃O₄ was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe₃O₄. The maximal SCFA production of the Fe₃O₄-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe₃O₄ was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe₃O₄ stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.

Enhanced solubilization and biochemical methane potential of waste activated sludge by combined free nitrous acid and potassium ferrate pretreatment

The increasing production of waste activated sludge (WAS) from wastewater treatment plants presents an inherent environmental burden. In this study, Free nitrous acid combined with potassium ferrate (FNA + PF) pretreatment was used to enhance solubilization and biochemical methane potential of WAS. Results indicated that the maximum removal rates of total suspended solid by PF, FNA, and PF + FNA pretreatment were 21.84%, 38.09%, and 56.17%, respectively. The biochemical methane potential of WAS without pretreatment reached 61.22 L CH₄/kg VSS added while this value increased to 147.07 L CH₄/kg VSS added after FNA + PF pretreatment (0.06 g/g TSS NaNO₂ and 0.25 g/g TSS K₂FeO₄). Shotgun metagenomic analysis revealed that FNA + PF pretreatment could increase the diversity and stability of microbial communities by shifting methanogenic pathways from strictly acetoclastic to acetoclastic/hydrogenotrophic, thereby enhancing methane production. This study suggested that FNA + PF pretreatment is a promising technology to reduce WAS and enhance methane production by pretreated WAS during anaerobic digestion.

Conductive materials in anaerobic digestion: From mechanism to application

Anaerobic digestion (AD) is an effective strategy combined advantages of maintaining the global carbon flux and efficient energy conversion. Various conductive materials (CMs) have been applied in anaerobic digesters to improve the performance of anaerobic fermentation and methanogenesis, including carbon-based CMs and metal-based CMs. Generally, CMs facilitated the AD thermodynamically and kinetically because they triggered more efficient syntrophic metabolism to increase electron capture capability and accelerate reaction rate as well as enhance the performance of AD stages (hydrolysis-acidification, methanogenesis). Besides, adding CMs into anaerobic digester is benefit to dealing with the deteriorating AD, which induced from temperature variation, acidified working condition, higher H₂ partial pressure, etc. However, few CMs exhibited inhibition on AD, including ferrihydrite, magnesium oxide, silver nanoparticles and carbon black. Inhibition comes from a series of complex factors, such as substrate competition, direct inhibition from Fe(III), Fe(III) reduction of methanogens, toxic effects to microorganisms and mass transfer limitation.