Enhancing sludge methanogenesis with improved redox activity of extracellular polymeric substances by hematite in red mud

Light alters microbiota and electron transport: Evidence for enhanced mesophilic digestion of municipal sludge

Light as an environmental factor can affect the process of anaerobic digestion, but there is no systematic study in municipal wastewater sludge mesophilic digestion. In this study, the effects of light on the performance of the anaerobic digestion system and photo-anaerobic microbiota (PAM) were evaluated in lighted anaerobic batch digesters (LABRs). The methane yield from the reactor under the dark condition (LABR0) was 179.2 mL CH₄/g COD, which was lower than 305.4 mL CH₄/g COD and 223.0 mL CH₄/g COD (n = 3, p < 0.05) from reactors under the light intensity of 3600 lm (LABR1) and 7200 lm (LABR2), respectively. The dominant genera in the bacterial and archaeal communities were Bacillus and Methanosarcina under light conditions, Enterococcus and Methanobacterium under dark conditions. And these two bacteria acted as electroactive bacterial genera, indicating that light changes the combination of direct interspecies electron transfer (DIET) microbial partners and activates the DIET pathway for methane production. The electron conduction pathways analysis further suggests that biological DIET (bDIET) between microbial biomass, rather than DIET via conductive material (cDIET) between microbes and conductive materials, is promoted and behaves as the dominant factor enhancing methane production under light conditions. The morphology of microorganisms and the amount and properties of EPS corroborate these views. Our findings are guided to anaerobic digester constructions under the outdoor environment with light exposure.

Enhancing methanogenesis of anaerobic granular sludge by incorporating Fe/Fe oxides nanoparticles aided with biofilm disassembly agents and mediating redox activity of extracellular polymer substances

Anaerobic granular sludge (AGS) is a promising technology for organic wastewater treatment and energy recovery. In this study, three different kinds of Fe and Fe oxides nanoparticles (Fe₃O₄, Fe₂O₃ and ZVI) were tried to be incorporated into AGS through direct loading or aided with biofilm disassembly agents of norspermidine and D-tyrosine, which was aimed to enhance methane production capacity of AGS via increasing redox activity of extracellular polymer substance (EPS) and interspecies electron transfer. Despite the loading methods, incorporation of Fe and Fe oxides nanoparticles into AGS increased methane production capacity remarkably, with an enhancement of 36.49-85.17%, 20.37-204.95% and 189.71-243.32%, respectively, for the Fe₃O₄, Fe₂O₃ and ZVI loaded AGS. Pretreatment of AGS using biofilm disassembly agents helped to incorporate more Fe and Fe oxides into the inner structure of AGS, which further enhanced methane production capacity by 48.68% and 184.58%, respectively, for the Fe₃O₄ and Fe₂O₃ loaded AGS. Loading Fe and Fe oxides into AGS not only introduced exogenous conductive substances and Fe(III)/Fe(II) redox couples into EPS matrix of AGS, but also stimulated the production of redox active components of flavins and c-Cyts. All these factors may contribute to the reduced resistance of EPS, enhanced interspecies electron transfer and methane production capacity of AGS. This study provides a novel strategy and facile method to accelerate interspecies electron transfer and enhance methane production for matured AGS.

Extracellular Polymeric Substances and Biocorrosion/Biofouling: Recent Advances and Future Perspectives

Microbial cells secrete extracellular polymeric substances (EPS) to adhere to material surfaces, if they get in contact with solid materials such as metals. After phase equilibrium, microorganisms can adhere firmly to the metal surfaces causing metal dissolution and corrosion. Attachment and adhesion of microorganisms via EPS increase the possibility and the rate of metal corrosion. Many components of EPS are electrochemical and redox active, making them closely related to metal corrosion. Functional groups in EPS have specific adsorption ability, causing them to play a key role in biocorrosion. This review emphasizes EPS properties related to metal corrosion and protection and the underlying microbially influenced corrosion (MIC) mechanisms. Future perspectives regarding a comprehensive study of MIC mechanisms and green methodologies for corrosion protection are provided.

Two-stage hybrid microalgal electroactive wetland-coupled anaerobic digestion for swine wastewater treatment in South China: Full-scale verification

Constructed wetlands have been widely used for organic wastewater treatment owing to low operating costs and simple maintenance. However, there are some disadvantages such as unstable efficiency in winter. In this study, a microalgal electroactive biofilm-constructed wetland was coupled with anaerobic digestion for full-scale treatment of swine wastewater. In a 12-month outdoor trial, the overall removal efficiencies of chemical oxygen demand, ammonium, nitrate, total nitrogen, total phosphorus, and nitrite reached 98.26%/95.14%, 97.96%/92.07%, 85.45%/66.04%, 95.07%/91.48%, 91.44%/91.52%, and 85.45%/84.67% in summer/winter, respectively. Hydrolytic bacteria were dominant in the anaerobic digestion part, and Cyanobium, Shewanella, and Azoarcus were enriched in the microalgal electroactive biofilm. The operating cost of the entire system was approximately 0.118 $/m³ of wastewater. These results confirm that the microalgal electroactive biofilm significantly enhances the efficiency and stability of constructed wetlands. In conclusion, the anaerobic digestion-microalgal electroactive biofilm-constructed wetland is technically and economically feasible for the treatment of swine wastewater.

Unraveling the roles of lanthanum-iron oxide nanoparticles in biohydrogen production

Low hydrogen (H₂) yield via dark fermentation often occurs, being mainly due to H₂ generation pathway shift. In this study, lanthanum-iron oxide nanoparticles (LaFeO₃ NPs) were prepared to investigate their effects on bioH₂ production. The highest H₂ yield of 289.8 mL/g glucose was found at 100 mg/L of LaFeO₃, being 47.6% higher than that from the control (196.3 mL/g glucose). The relative abundance of Firmicutes increased from 54.2% to 67.5%. The large specific surface area of LaFeO₃ provided sufficient sites for the colonization of Firmicutes and increased the bacterial access to nutrients. Additionally, the La³⁺ gradually released from LaFeO₃ NPs raised microbial transmembrane transport capacity, promoting glycolytic efficiency and Fe availability, thereby increasing hydrogenase content, and shifting the bioH₂ evolution to butyrate pathway for more H₂. This provides the novelty for biochemical utilization of La and new insights into the improved H₂ yield amended with LaFeO₃.

Promoting biomethane production from propionate with Fe2O3@carbon nanotubes composites

Using a batch anaerobic system constructed with 60 mL serum bottles, potential of a composite material with Fe₂O₃ nanoparticles decorated on carbon nanotubes (CNTs) to enhance biomethane production was investigated. The composites (Fe₂O₃@CNTs) with well dispersed Fe₂O₃ nanoparticles (4.5 nm) were fabricated by a facile thermal decomposition method in a muffle furnace under nitrogen atmosphere. Compared with Fe₂O₃, Fe₂O₃@CNTs showed a large specific surface area and good electrical conductivity. Supplementation of Fe₂O₃@CNTs to the propionate-degrading enrichments enhanced the methane production rate, which was 10.4-fold higher than that in the control experiment without material addition. The addition of Fe₂O₃@CNTs also not only showed a clearly electrochemical response to flavin and cytochrome C, but also reduced the electron transfer resistance when compared to the control. Comparative analysis showed that Fe₂O₃ in Fe₂O₃@CNTs played a key role in initiating electrochemical response and triggering rapid methane production, while CNTs functioned as rapid electron conduits to facilitate electron transfer from iron-reducing bacteria (e.g., Acinetobacter, Syntrophomonas, and Geobacter) to methanogens (e.g. Methanosarcina).

Response of methanogenic granules enhanced by magnetite to ammonia stress

Excessive ammonia has an inhibitory effect on anaerobic granular sludge (AnGS) when treating industrial wastewater with high concentration of ammonia and organic matters. The addition of conductive materials has been widely reported to improve the AnGS activity, which has the potential to alleviate the ammonia inhibition. In this study, the addition of magnetite was carried out to enhance the activity of AnGS in UASB reactor, then the response of AnGS to different ammonia levels was investigated. Results showed that magnetite facilitated the enrichment of Methanosaeta and Clostridium sensu stricto 1. Under the ammonia stress (up to 5 g TAN/L), it was interesting that Methanosaeta was better retained (abundance of 45.8%), and the abundance of ammonia-resistant Clostridium sensu stricto 1 increased to 34.3% in presence of magnetite. RT-qPCR analysis revealed that Methanosaeta could maintain metabolically active for counteracting the ammonia inhibition along with the higher transcription of genes encoding for CO₂-dependent pathway. The electron transport activity and ATP content of AnGS were 1.25-2.12 and 1.23-2.56 folds higher than those in the control group, respectively. In addition, the AnGS could maintain the stability of structure because Methanosaeta was the skeleton of AnGS. As a result, the analysis of enzyme activity showed that the overall methanogenic metabolism was more active, thus ensured the effective operation of UASB reactor. This study provided the scientific understanding about the role of magnetite to alleviate the ammonia inhibition, and had important implications for stable treatment and recycling of industrial wastewater.

Immobilization and redistribution process of As(V) during As(V)-bearing ferrihydrite reduction by Geobacter sulfurreducens under the influence of TiO2 nanoparticles

The redistribution process of arsenate (As(V)) and the variation in As(V) content in different locations must be clarified to ensure low mobility of As(V) during microbial ferrihydrite reduction. In this study, we investigated As(V) immobilization and redistribution processes when ferrihydrite was incubated with Geobacter sulfurreducens in the presence of titanium dioxide (TiO₂) nanoparticles. Our study results showed that, As(V) in the aqueous phase and ferrihydrite were redistributed on light minerals (goethite), heavy minerals (ferrihydrite and magnetite), and extracellular polymeric substances (EPS) induced by G. sulfurreducens during ferrihydrite reduction. Interestingly, we found that As(V) in the form of arsenate ion (AsO₄^3-) was adsorbed by the functional groups of the EPS, while the formed Fe^II₃(As^VO₄)₂ was wrapped in the network structure of EPS. Moreover, the addition of TiO₂ nanoparticles did not promote but delayed the entire ferrihydrite reduction, As(V) immobilization and redistribution processes. Furthermore, changes in the aqueous arsenic and iron concentrations are closely related to the formation time of secondary minerals. Our study findings provide new insights into the As(V) immobilization process mediated by G. sulfurreducens under anaerobic conditions.

Exopolysaccharides matrix affects the process of extracellular electron transfer in electroactive biofilm

The applications of bioelectrochemical systems (BESs) in the field of environment and energy are achieved through the bioelectrocatalytic process of electroactive biofilms. As a primary component of biofilm, the role of exopolysaccharides in electroactive biofilm in BESs is poorly understood. This study constructed an exopolysaccharides-deficient Geobacter sulfurreducens-based BES to explore the role of exopolysaccharides in electroactive biofilm. Compared with the wild type, the mutant biofilm expressing less exopolysaccharides decreased the capacity of current generation. In the mutant biofilm, the content of exopolysaccharides decreased significantly, resulting in a thinner biofilm and lower cell viability compared with the wild-type biofilm. However, the mutant with overexpressed pili developed a mature biofilm with extended time, which indicating the importance of exopolysaccharides for early biofilm formation and the compensatory role of pili in biofilm formation. The mutant biofilm had less content of c-type cytochromes (c-Cyts) and lower electrochemical activity of extracellular polymeric substances than the wild-type biofilm, suggesting a function of exopolysaccharides anchoring extracellular c-Cyts that essential to extracellular electron transfer (EET) in electroactive biofilms. Our findings demonstrated the essential role of exopolysaccharides in the process of EET in electroactive biofilm, which contributed to a better understanding and optimization of the performance of BESs.

Use of extracellular polymeric substances as natural redox mediators to enhance denitrification performance by accelerating electron transfer and carbon source metabolism

Extracellular polymeric substances (EPS) extracted from waste activated sludge were used as endogenous redox mediator to enhance denitrification performance. The nitrate reduction rate increased 1.42-fold when EPS were added at 75 mg C/L (C represents total organic carbon). EPS addition decreased the charge transfer resistance and improved the electron transport system activity. The nitrate reductase and nitrite reductase activities improved by 29.7% and 25.4%, respectively. The activation energy of the system with EPS addition (25.82 kJ/mol) was 31.1% lower than that of the control group (37.49 kJ/mol). Besides, EPS could be used as electron carriers to accelerate electron transport; its primary role was similar to that of the quinone loop in the electron transfer chain. More importantly, EPS addition enhanced carbon source metabolism, which increased the available nicotinamide adenine dinucleotide yield to 1.21 times that of the control group, and thus promoted the denitrification performance of activated sludge.

Effects of ferroferric oxide on azo dye degradation in a sulfate-containing anaerobic reactor: From electron transfer capacity and microbial community

Anaerobic decolorization of azo dye in sulfate-containing wastewater has been regarded as an economical and effective method, but it is generally limited by the high concentration of azo dye and accumulation of toxic intermediates. To address this problem, Fe₃O₄ was added to one of the anaerobic reactors to investigate the effects on system performances. Results showed that AO7 removal rate, COD removal rate, and sulfate reduction were enhanced with the addition of Fe₃O₄ under various influent AO7 concentrations (153 mgCOD/L - 1787 mgCOD/L). According to the proposed pathway for the degradation of AO7, more intermediates (2-hydroxy-1,4-naphthoquinone, phthalide, 4-methylphenol) were produced in the presence of Fe₃O₄. The electron transfer capacity of sludge was also increased since Fe₃O₄ could stimulate to secrete humic acid-like organics in EPS. Microbial analysis showed that iron-reducing bacteria like Clostridium and Geobacter were also enriched, which were capable of azo dye and aromatic compounds degradation.

Mechanism study of improving anaerobic co-digestion performance of waste activated sludge and food waste by Fe3O4

A large amount of waste activated sludge (WAS) and food waste (FW) are produced every year in China. Anaerobic co-digestion is considered to be an effective way to solve this problem. This study applied FW/WAS mixture as co-substrate to create different digestive environment, aiming to understand the mechanism of Fe₃O₄ particles in promoting AD performance. The results showed that the addition of Fe₃O₄ presented various performances when facing different digestive acidification stress brought by different mixing ratios of WAS and FW. Methanogenic pathways and microbial communities varied with substrates' properties. For group A (WAS mono-digestion), the acetoclastic methanogens dominated, 20 mg/g VS (according to the iron element) Fe₃O₄ could promote methane production, while 200 mg/g VS Fe₃O₄ would inhibit microbial activity. The promoted methane production by Fe₃O₄ was attributable to the promotion of sludge hydrolysis. For group B (WAS: FW = 1:0.5, based on VS addition, similarly hereinafter), Fe₃O₄ triggered direct interspecific electron transfer (DIET) between bacteria and methanogens. For group C (WAS: FW = 1:1), the hydrogenotrophic methanogens dominated, bacteria excreted more non-conductive polysaccharides in EPS to resist unfavorable environment, thereby it prevented their contact with Fe₃O₄ particles. So, it was difficult for Fe₃O₄ to trigger DIET and promote the digestive performance of batch experiments in such condition.

Buffer capacity regulates the stratification of anode-respiring biofilm during brewery wastewater treatment

Improving the buffer capacity of the electrolyte can enhance the anode performance in bioelectrochemical systems (BESs). To elucidate the mechanism underlying the facilitated BESs performance, this study used three different anode biofilms cultured with different concentrations of phosphate buffer (5, 50 and 100 mM) to investigate the biofilm response, in terms of the spatial structure of metabolic activity and microbial community, to different buffer capacities. Results showed that the electrochemical activities of the anode biofilms positively correlated with the buffer concentration. The spatial stratification of metabolic activity and microbial community of the anode biofilms were regulated by the buffer capacity, and the spatial microbial heterogeneity of the anode biofilm decreased as the buffer concentration increased. With increasing buffer capacity, Geobacter spp. were enriched in both the inner and outer layers of the biofilm, and the inhibition of methanogens growth improved the COD removal attributed to anode respiration. Additionally, the stimulation of EPS production in biofilms played a role in increasing the electrochemical performance of biofilms by buffer improvement. This study first revealed the regulation of buffer capacity on the stratification of anode biofilm during brewery wastewater treatment, which provided a deep insight into the relation of biofilm structure to its electrochemical properties.

Extracellular electron transfer via multiple electron shuttles in waterborne Aeromonas hydrophila for bioreduction of pollutants

Members of the genus Aeromonas prevail in aquatic habitats and have a great potential in biological wastewater treatment because of their unique extracellular electron transfer (EET) capabilities. However, the mediated EET mechanisms of Aeromonas have not been fully understood yet, hindering their applications in biological wastewater treatment processes. In this study, the electron shuttles in Aeromonas hydrophila, a model and widespread strain in aquatic environments and wastewater treatment plants, were explored. A. hydrophila was found to produce both flavins and 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ) as electron shuttles and utilize them to accelerate its EET for the bioreduction of various pollutants. The Mtr-like respiratory pathway was essential for the reduction of flavins, but not involved in the ACNQ reduction. The electron shuttle activity of ACNQ for pollutant bioreduction involved the redox reactions that occurred inside the cell. These findings deepen our understanding about the underlying EET mechanisms in dissimilatory metal reducing bacteria and provide new insights into the roles of the genus Aeromonas in biological wastewater treatment.

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.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.

Zeolitic imidazolate framework-derived porous carbon enhances methanogenesis by facilitating interspecies electron transfer: Understanding fluorimetric and electrochemical responses of multi-layered extracellular polymeric substances

Modulating microbial electron transfer during anaerobic digestion can significantly improve syntrophic interactions for enhanced biogas production. As a carbonaceous conductive material, zeolite imidazolate framework-67 (ZIF-67)-derived porous carbon (PC) was hypothesized to act as a microbial electron transfer highway and assessed with respect to understanding the fluorimetric and electrochemical responses of multilayered extracellular polymeric substances (EPS). The highest biomethane yield (614.0 mL/g) from ethanol was achieved in the presence of 100 mg/L PC prepared at a carbonization temperature of 800 °C (PC-800), which was 28.2% higher than that without PC addition. Electrochemical analysis revealed that both the redox peak currents and conductivity of the methanogenic sludge increased, while the free charge transfer resistance decreased with PC-800 addition. The conductive PC-800 potentially functioned as an abiotic electron conduit to promote direct interspecies electron transfer, thereby resulting in decreased expression of functional genes associated with electrically conductive pili (e-pili) and hemeproteins. Additionally, PC-800 stimulated the secretion of redox-active humic substances (HSs), and excitation emission matrix spectra analysis indicated that the largest increase in percent fluorescence response of HSs occurred in the tightly bound EPS (TB-EPS) with addition of PC-800. This was attributed to the strong complexation ability of PC-800 particles to hydroxyl/carboxylic/phenolic moieties of HSs contained in the TB-EPS. Microbial analysis revealed that syntrophic/exoelectrogenic bacteria such as Pelotomaculum and Syntrophomonas, as well as hydrogenotrophic/electrotrophic methanogens such as Methanoculleus and Methanobacterium, were enriched in methanogenic sludge with adding PC-800. This study provided comprehensive insights for understanding the interactions among ZIF-derived PC, methanogenic microorganisms and their multilayered EPS.

Comparison of mesophilic and thermophilic dark fermentation with nickel ferrite nanoparticles supplementation for biohydrogen production

In this work, nickel ferrite nanoparticles (NiFe₂O₄ NPs) was prepared to improve hydrogen (H₂) production by dark fermentation. Moderate amounts (50-200 mg/L) promoted H₂ generation, while excess NiFe₂O₄ NPs (over 400 mg/L) lowered H₂ productivity. The highest H₂ yields of 222 and 130 mL/g glucose were obtained in the 100 mg/L (37 °C) and 200 mg/L NiFe₂O₄ NPs (55 °C) groups, respectively, and the values were 38.6% and 28.3% higher than those in the control groups (37 °C and 55 °C). Soluble metabolites showed that NiFe₂O₄ NPs enhanced the butyrate pathway, corresponding to the increased abundance of Clostridium butyricum in mesophilic fermentation. The endocytosis of NiFe₂O₄ NPs indicated that the released iron and nickel favored ferredoxin and hydrogenase synthesis and activity and that NiFe₂O₄ NPs could act as carriers in intracellular electron transfer. The NPs also optimized microbial community structure and increased the levels of extracellular polymeric substances, leading to increased H₂ production.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.

Exemplifying an archetypal thorium-EPS complexation by novel thoriotolerant Providencia thoriotolerans AM3

It is the acquisition of unique traits that adds to the enigma of microbial capabilities to carry out extraordinary processes. One such ecosystem is the soil exposed to radionuclides, in the vicinity of atomic power stations. With the aim to study thorium (Th) tolerance in the indigenous bacteria of such soil, the bacteria were isolated and screened for maximum thorium tolerance. Out of all, only one strain AM3, found to tolerate extraordinary levels of Th (1500 mg L⁻¹), was identified to be belonging to genus Providencia and showed maximum genetic similarity with the type strain P. vermicola OP1T. This is the first report suggesting any bacteria to tolerate such high Th and we propose to term such microbes as ‘thoriotolerant’. The medium composition for cultivating AM3 was optimized using response surface methodology (RSM) which also led to an improvement in its Th-tolerance capabilities by 23%. AM3 was found to be a good producer of EPS and hence one component study was also employed for its optimization. Moreover, the EPS produced by the strain showed interaction with Th, which was deduced by Fourier Transform Infrared (FTIR) spectroscopy.

Coordinated response of Au-NPs/rGO modified electroactive biofilms under phenolic compounds shock: Comprehensive analysis from architecture, composition, and activity

Electroactive biofilms (EABs) can be integrated with conductive nanomaterials to boost extracellular electron transfer (EET) for achieving efficient waste treatment and energy conversion in bioelectrochemical systems. However, the in situ nanomaterial-modified EABs of mixed-culture, and their response under environmental stress are rarely revealed. Here, two nanocatalyst-decorated EABs were established by self-assembled Au nanoparticles-reduced graphene oxide (Au-NPs/rGO) in mixed-biofilms with different maturities, then their multi-property were analyzed under long-term phenolic shock. Results showed that the power density of Au-NPs/rGO decorated EABs was significantly enhanced by 28.66-42.82% due to the intensified EET pathways inside biofilms. Meanwhile, the electrochemical and catalytic performance of EABs were controllably regulated by 0.3-3.0 g/L phenolic compounds, which, however, resulted in differential alterations in their architecture, composition, and viability. EABs originated with higher maturity displayed more compact structure, lower thickness (110 μm), higher biomass (8.67 mg/cm²) and viability (0.85-0.91), endowing it better antishock ability to phenolic compounds. Phenolic-shock also induced the heterogeneous distribution of extracellular polymeric substances in terms of both spatial and bonding degrees of the decorated EABs, which could be regarded as an active response to strike a balance between self-protection and EET under environmental pressure. Our findings provide a broader understanding of microbe-electrode interactions in the micro-ecology interface and improve their performance in the removal of complex contaminants for sustainable remediation and new-energy development.

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces

Electrochemically active bacteria can transport their metabolically generated electrons to anodes, or accept electrons from cathodes to synthesize high-value chemicals and fuels, via a process known as extracellular electron transfer (EET). Harnessing of this microbial EET process has led to the development of microbial bio-electrochemical systems (BESs), which can achieve the interconversion of electrical and chemical energy and enable electricity generation, hydrogen production, electrosynthesis, wastewater treatment, desalination, water and soil remediation, and sensing. Here, the focus is on the current understanding of the microbial EET process occurring at both the bacteria-electrode interface and the biotic interface, as well as some attempts to improve the EET by using various nanomaterials. The behavior of nanomaterials in different EET routes and their influence on the performance of BESs are described. The inherent mechanisms will guide rational design of EET-related materials and lead to a better understanding of EET mechanisms.

Enhancing inhibition of disinfection byproducts formation and opportunistic pathogens growth during drinking water distribution by Fe2O3/Coconut shell activated carbon

The effects of biological activated carbon treatment using Fe₂O₃ modified coconut shell-based activated carbon (Fe/CAC) were investigated on the occurrence of opportunistic pathogens (OPs) and formation of disinfection by-products (DBPs) in simulated drinking water distribution systems (DWDSs) with unmodified CAC as a reference. In the effluent of annular reactor (AR) with Fe/CAC, the OPs growth and DBPs formation were inhibited greatly. Based on the differential pulse voltammetry and dehydrogenase activity tests, it was verified that extracellular electron transfer was enhanced in the attached biofilms of Fe/CAC, hence improving the microbial metabolic activity and biological removal of organic matter especially DBPs precursors. Meanwhile, the extracellular polymeric substances (EPS) on the surface of Fe/CAC exhibited stronger viscosity, higher flocculating efficiency and better mechanical stability, avoiding bacteria or small-scale biofilms falling off into the water. Consequently, the microbial biomass and EPS substances amount decreased markedly in the effluent of Fe/CAC filter. More importantly, Fe/CAC did significantly enhance the shaping role on microbial community of downstream DWDSs, continuously excluding OPs advantage and inhibiting EPS production. The weakening of EPS in DWDSs resulted in decrease of microbial chlorine-resistance ability and EPS-derived DBPs precursors supply. Therefore, the deterioration of water quality in DWDSs was inhibited greatly, sustainably maintaining the safety of tap water. Our findings indicated that optimizing biological activated carbon treatment by interface modification is a promising method for improving water quality in DWDSs.

Understanding the key regulatory functions of red mud in cellulose breakdown and succession of β-glucosidase microbial community during composting

The purpose of this research was to explore the effects of red mud on cellulose degradation and the succession of β-glucosidase microbial community in composting to better enhance the quality of compost. The activity of β-glucosidase in the treatment group with red mud (T) was 0.42-1.07 times higher than that in the control group without red mud (CK) from day 7 to 21 of composting. The final cellulose degradation ratios of the T (84.73%) were 10.02% higher than that of the CK (74.71%). In addition, Proteobacteria, Actinobacteria, Firmicutes, and Ascomycota were the most dominant β-glucosidase-producing microbes, and these microbes were also the phyla causing composting performances differences in the high temperature, cooling, and maturity periods of CK and T. These results indicated that adding red mud can improve β-glucosidase activity and boost the breakdown of cellulose in composting process.

Extracellular polymeric substances trigger an increase in redox mediators for enhanced sludge methanogenesis

Artificial redox mediators can be employed to improve the electron transfer efficiency during sludge methanogenesis, whereas these artificial redox mediators have possible deficiencies, such as high cost and non-biodegradability. For large-scale commercial applications, more cost-effective and environmentally friendly alternatives should be developed. Herein, the potential of extracellular polymeric substances (EPS) as natural redox mediators to improve methanogenesis was investigated. Compared to the control test without EPS addition, the methane (CH₄) production yield was increased by 83.5 ± 2.4% with an EPS dosage of 0.50 g/L and the lag phase duration was shortened by 45.6 ± 7.0%, along with the enhanced sludge dewaterability. Spectroelectrochemical measurements implied that EPS addition notably changed the intensities of different redox-active groups, which decreased the charge transfer resistance and enhanced the extracellular electron transfer efficiency. These redox-active groups were mainly from the solubilization and hydrolysis of sludge protein due to increased protease activities, thereby leading to a higher acetate concentration during the acidification step. Further investigation showed that EPS addition also improved the activities of both acetotrophic and hydrogenotrophic methanogens, as indicated by a higher abundance of alpha subunit of methyl coenzyme M reductase (mcrA) genes, enhancing CH₄ production. This work provides an innovative strategy for improving sludge anaerobic digestion with efficient additives.

Dual roles of zero-valent iron in dry anaerobic digestion: Enhancing interspecies hydrogen transfer and direct interspecies electron transfer

Although commonly viewed as a promising method, dry anaerobic digestion is not been widely applied to dispose of food wastes, especially in developing countries because of its insufficiency in handling with lower mass transfer and high acidic accumulation of the system. Zero valent iron (ZVI) has been found to demonstrate superior performance such as enhancing methane production. However, up to date, the mechanism of ZVI remains unclear. In this study, adding 5 g/L ZVI could improve interspecies hydrogen transfer (IHT) to enhance the dry anaerobic digestion of food wastes, but was unable to resist the shocks of high organic loading. With increasing ZVI dosage to 10 g/L, the performances of digestion systems were improved to maintain the systems stable. With 10 g/L of ZVI addition, electron transfer capacity of the sludge increased by 5.4 folds, and electroactive proteins of sludge increased by 2.3 folds. Microbial community analysis also indicated that the relative abundances of Methanothrix and Methanosarcina performing direct interspecies electron transfer were enriched to 67.5% and 27.2% with 10 g/L ZVI addition, respectively. These results suggested that direct interspecies electron transfer could be established with a proper dosage of ZVI that served as a conductive material to connect electron exchange among microorganisms. Thus, ZVI played a dual role including improving interspecies hydrogen transfer and promoting direct interspecies electron transfer to keep the systems efficient to treat high-solid food wastes.

Advanced bioH2 and bioCH4 production with cobalt-doped magnetic carbon

In this work, a novel cobalt-doped magnetic carbon (CDMC) was prepared to boost hydrogen (H₂) and methane (CH₄) generation. A one-pot approach was employed to produce H₂ and CH₄ with an incompletely heat-treated mixed culture. A moderate amount of CDMC promoted biogas evolution, while excess CDMC eroded both H₂ and CH₄ productivity. The CDMC (600 mg L⁻¹) group achieved the highest biogas yields of 176 mL H₂ per g glucose and 358 mL CH₄ per g glucose, which were higher than those (102 mL H₂ per g glucose and 288 mL CH₄ per g glucose) found in the control group without CDMC. The mechanisms of H₂ and CH₄ production via the one-pot approach with CDMC were speculated to be as follows: CDMC provided beneficial sites and two elements (Co and Fe) for culture growth and boosted electron transfer, facilitating glucose degradation and conversion. Supplementation of carbon matrix composites and trace elements in biogas production has been shown to be an efficient strategy.

In this work, a novel cobalt-doped magnetic carbon (CDMC) was prepared to boost hydrogen (H₂) and methane (CH₄) generation.

Perchlorate bioreduction by anaerobic granular sludge immobilised with Fe-HA complex: Performance, extracellular polymeric substances and microbial community structure

An iron-humic acid (Fe-HA) complex was used as a redox mediator in perchlorate (ClO₄^-) bioreduction. Bioreduction performance, extracellular polymeric substances (EPS), and microbial community structure were comprehensively explored using different types of anaerobic granular sludge (AnGS) immobilised without the Fe-HA complex (AnGS_CON) and with the Fe-HA complex (AnGS_FH). The ClO₄^- was completely removed by AnGS_CON by day 20, while the ClO₄^- was completely removed by AnGS_FH by day 6. The AnGS immobilised with the Fe-HA complex significantly increased the ClO₄^- bioreduction. The acceleration of ClO₄^- bioreduction was also explained by the mixed liquor volatile suspended solids (MLVSS), MLVSS/mixed liquor suspended solids (MLSS), EPS composition, and microbial community structure. Compared with AnGS_CON, the MLVSS and MLVSS/MLSS of the AnGS_FH increased 1.4- and 1.2-fold, respectively. Humic substances in the EPS were stimulated by the Fe-HA complex. The microbial community structure analysis indicated that perchlorate and quinone reducing bacteria were enriched by the Fe-HA complex. Based on the analysis, the ClO₄^- bioreduction mechanism of the AnGS_FH was revealed because the Fe-HA complex in the AnGS increased the biomass concentration, biological activity, and redox-active mediator and shifted the microbial community structure.

Red mud-based inorganic polymer spheres: Innovative and environmentally friendly anaerobic digestion enhancers

Red mud-based inorganic polymer spheres were used as alternative pH regulators and process enhancers in sequencing batch anaerobic reactors treating cheese whey. This byproduct tends to quickly acidify under anaerobic conditions, and the common route to control pH and ensure suitable conditions for methane production involves the use of commercial alkaline raw materials. The spheres were synthesized using significant amounts of hazardous and toxic waste, red mud (50 wt% of solid components), whose recycling is challenging. The inorganic polymeric spheres, when compared to virgin alkaline raw materials, improved organic matter removal by 44%, prevented VFA accumulation (acidification degree less than 20%), maintained pH values in a range (6.5-7.2) to ensure maximum methanogenic activity by archaea microorganisms, and boosted the methane volume by ~90%. These promising results demonstrate the feasibility and performance advantages of using these innovative spheres instead of virgin raw materials, which is an important tool towards sustainable development.

Cadmium sulfide nanoparticles-assisted intimate coupling of microbial and photoelectrochemical processes: Mechanisms and environmental applications

Intimate coupling of microbial extracellular electron transfer (EET) and photoelectrochemical processes is an emerging research area with great potential to circumvent many disadvantages associated with traditional techniques that depend on independent microbial or photocatalysis treatment. Microbial EET processes involve microorganism oxidation of extracellular electron donors for respiration and synchronous reduction of extracellular electron acceptors to form an integrated respiratory chain. Coupled microbial EET-photoelectrochemical technologies greatly improve energy conversion efficiency providing both economic and environmental benefits. Among substitutes for semiconductor photocatalysts, cadmium sulfide nanoparticles (CdS NPs) possess several attractive properties. Specifically, CdS NPs have suitable electrical conductivity, large specific surface area, visible light-driven photocatalysis capability and robust biocompatibility, enabling them to promote hybrid microbial-photoelectrochemical processes. This review highlights recent advances in intimately coupled CdS NPs-microbial extracellular electron transfer systems and examines the mechanistic pathways involved in photoelectrochemical transformations. Finally, the prospects for emerging applications utilizing hybrid CdS NPs-based microbial-photoelectrochemical technologies are assessed. As such, this review provides a rigorous fundamental analysis of electron transport dynamics for hybrid CdS NPs-microbial photoelectrochemical processes and explores the applicability of engineered CdS NPs-biohybrids for future applications, such as in environmental remediation and clean-energy production.

Effects of different hypochlorite types on the waste activated sludge fermentation from the perspectives of volatile fatty acids production, microbial community and activity, and characteristics of fermented sludge

The effects of different hypochlorite types (namely Ca(OCl)₂ and NaOCl) on the waste activated sludge (WAS) anaerobic fermentation, and microbial community and activity were investigated. The results indicated that both Ca(OCl)₂ and NaOCl contributed to volatile fatty acids (VFAs) production by simultaneously enhancing the solubilization, hydrolysis and acidification processes. The maximal VFAs was respectively 1379.5 (at 10 d) and 1621.5 (at 8 d) mg COD/L at the optimal dose of NaOCl and Ca(OCl)₂ while it was merely 157.4 (at 6 d) mg COD/L in the control. However, the Ca(OCl)₂ might affect the anaerobic process in a continuous mode while the NaOCl was relatively transient, which caused distinctive influences on the microbial structure and activity, and subsequently VFAs production in WAS fermentation systems. Moreover, Ca(OCl)₂ treatments showed advantages over NaOCl on WAS dewatering and VSS reduction, implying the superiority of utilizing Ca(OCl)₂ as additives for WAS disposal.

Variation on leaching behavior of caustic compounds in bauxite residue during dealkalization process

Bauxite residue, a byproduct of alumina manufacture, is a serious environmental pollutant due to its high leaching contents of metals and caustic compounds. Four typical anions of CO₃^2-, HCO₃^-, Al(OH)₄^- and OH^- (represented caustic compounds) and metal ions (As, B, Mo and V) were selected to assess their leaching behavior under dealkalization process with different conditions including liquid/solid ratio (L/S ratio), temperature and leaching time. The results revealed that washing process could remove the soluble composition in bauxite residue effectively. The leaching concentrations of typical anions in bauxite residue decreased as follows: c(CO₃^2-) > c(HCO₃^-) > c[Al(OH)₄^-] > c(OH^-). L/S ratio had a more significant effect on leaching behavior of OH^-, whilst the leaching concentration of Al(OH)₄^- varied larger underleaching temperature and time treatment. Under the optimal leaching, the total alkaline, soluble Na concentrations, exchangeable Ca concentrations were 79.52, 68.93, and 136.0 mmol/L, respectively, whilst the soluble and exchangeable content of As, B, Mo and V in bauxite residue changed slightly. However, it should be noted that water leaching has released metal ions such as As, B, Mo and V in bauxite residue to the surrounding environment. The semiquantitative analysis of XRD revealed that water leaching increased the content of gismondine from 2.4% to 6.4%. The SEM images demonstrated the dissolution of caustic compounds on bauxite residue surface. The correlation analysis indicated that CO₃^2- and HCO₃^- could effectively reflect the alkalinity of bauxite residue, and may be regarded as critical dealkalization indicators to evaluate alkalinity removal in bauxite residue.

Humification in extracellular polymeric substances (EPS) dominates methane release and EPS reconstruction during the sludge stabilization of high-solid anaerobic digestion

High-solid anaerobic digestion (HAD) can directly treat dewatered sewage sludge (total solid content ≥15%) with superior volume efficiency. Sludge stabilization during HAD is expected to achieve by throughout organic degradation and conversion towards methane-rich biogas release and humic formation. Sewage sludge is the combination of microbial zoogleas and theirs adsorption of organic and inorganic matter, in which the extracellular polymeric substances (EPS) account 60-80% of total sludge organic matter, inevitably participating most extracellular metabolic pathways. The interactions between EPS transformation and genetically annotated metabolic pathways were found in this research. In brief, noticing the highly cross-linked structures in EPS with major active components of humic substances (HS) and protein (PN), as PN hydrolysis and decomposition in EPS were enhanced in the high-solid anaerobic condition, the exposure of aromatic groups and sites in HS were considerable. HS release was the main factor shifting the electron exchange capacity and activity, which aided in energy metabolism of sludge microorganisms involved in redox reactions, especially the methanogenesis, thus in turn facilitating the PN degradation; Then, the screened humic groups and active protein derives might act as the beneficial precursors to regenerate neo-humic structures, whose significant bridging effect and signal role on stimulating amino acid biosynthesis, member transport and metallic complexation could further contribute to proteolytic condensation and EPS reconstruction. Hence, the in-depth sludge stabilization mechanism during HAD process was established for developing enlightening strategies.

Characterization of novel thorium tolerant Ochrobactrum intermedium AM7 in consort with assessing its EPS-Thorium binding

Currently, radioactive waste is disposed primarily by burial in a deep geological repository. Microorganisms thriving in such contaminated environment show tolerance to radionuclides. In the present study the bacterial flora, from soil sample collected from an area around atomic power station exposed to radionuclides and heavy metals, was cultivated and assessed for thorium (Th) tolerance. Of all the isolates, strain AM7 identified as O. intermedium was selected since it could thrive at high levels of Th (1000 mg L^-1). AM7 was characterized physico-chemically and its culture medium was optimized using central composite design of response surface methodology for assessing its growth properties in presence of Th. The strain also showed exceptional exopolysaccharide (EPS) production and its yield was further analyzed using one factor study to investigate the influence of each medium component. On supplementing the EPS medium with Th, no significant decrease in yield was observed. FTIR spectroscopy revealed the functional groups of EPS involved in EPS-Th binding. To the best of our knowledge, this is the first report showing exceptional Th-tolerance by any bacteria. Such study will help other researchers to strategize an environment-friendly way of radwaste disposal.

Enhanced Anaerobic Digestion by Stimulating DIET Reaction

Since the observation of direct interspecies electron transfer (DIET) in anaerobic mixed cultures in 2010s, the topic “DIET-stimulation” has been the main route to enhance the performance of anaerobic digestion (AD) under harsh conditions, such as high organic loading rate (OLR) and the toxicants’ presence. In this review article, we tried to answer three main questions: (i) What are the merits and strategies for DIET stimulation? (ii) What are the consequences of stimulation? (iii) What is the mechanism of action behind the impact of this stimulation? Therefore, we introduced DIET history and recent relevant findings with a focus on the theoretical advantages. Then, we reviewed the most recent articles by categorizing how DIET reaction was stimulated by adding conductive material (CM) and/or applying external voltage (EV). The emphasis was made on the enhanced performance (yield and/or production rate), CM type, applied EV, and mechanism of action for each stimulation strategy. In addition, we explained DIET-caused changes in microbial community structure. Finally, future perspectives and practical limitations/chances were explored in detail. We expect this review article will provide a better understanding for DIET pathway in AD and encourage further research development in a right direction.

Insights into compositional changes of dissolved organic matter during a full-scale vermicomposting of cow dung by combined spectroscopic and electrochemical techniques

Various spectroscopic and electrochemical techniques combined was used to investigate the compositional changes of dissolved organic matter (DOM) and the difference in humification degree during full-scale cow dung vermicomposting. This study also investigated that whether the two techniques could be used as humification indices. The physicochemical characteristics of vermicompost were superior to those of the control, indicating that vermicomposting significantly accelerated the humification process, which was confirmed by spectroscopic and electrochemical analyses. Meanwhile, the changes of three components identified and electron transfer capacities in vermicomposting further revealed that vermicomposting resulted in significant compositional changes of DOM and higher humification degree. Partial least squares path modeling and redundancy analysis revealed that the two techniques could be used as humification indices for vermicomposting. These results of this study demonstrated that the combination of spectroscopy and electrochemistry was applicable to characterize the compositional changes of DOM and the humification degree of vermicomposting.

Enhanced reduction of sulfate and chromium under sulfate-reducing condition by synergism between extracellular polymeric substances and graphene oxide

Microbial reduction of sulfate and metal were simultaneously enhanced in the presence of graphene oxide (GO)-like nanomaterials, however, the mechanism remained unclear. In this study, bio-reduction of Cr was compared between free-living bacterium BY7 and immobilized BY7 (BY-rGO) on reduced GO particles. The role of extracellular polymeric substances (EPS) and rGO material on reduction of sulfate and Cr was investigated. Cr(VI) was reduced to Cr(III) and elemental Cr by BY-rGO particles up to 51% and 28%, respectively. EPS produced by the bacterium BY7 mainly consisted of proteins, polysaccharides, nucleic acids and humic substances. Concentration of EPS was sharply increased (about 54%) with the addition of graphene oxide, while the composition of EPS components was strongly affected by the exposure to Cr. By removing surface EPS without breaking the cells, reduction activities of sulfate and chromium by both BY-rGO particles and free-living BY7 cells were decreased. In contrast, reduction of sulfate and Cr by the free-living BY7 cells was enhanced with external addition of extracted EPS. Based on electrochemical analysis, the reduction peak indicating enhanced electron transfer was lost after removing EPS. Moreover, the contribution of each EPS fractions on sulfate and Cr reduction followed an order of polysaccharides > proteins > humic substances. Therefore, microbial sulfate and Cr reduction processes in the presence of BY-rGO particles were enhanced by the increasing amounts of EPS, which likely mediated electron transfer during sulfate and Cr reduction, and relieved bacteria from metal toxicity. Nevertheless, the presence of rGO was crucially important for elemental Cr production under sulfate-reducing condition, which might contribute to lowering electric potential or reducing activation energy for Cr(III) reduction. This work provided direct evidences for enhancing sulfate and Cr reduction activities by supplement of EPS as an additive to increase treatment efficiency in environmental bioremediation.

Phosphate recovery with granular acid-activated neutralized red mud: Fixed-bed column performance and breakthrough curve modelling

Granular acid-activated neutralized red mud (AaN-RM) has been successfully prepared with good chemical stability and physical strength. However, its potential for industrial application remains unknown. Therefore, the performance of granular AaN-RM for phosphate recovery in a fixed-bed column was investigated. The results demonstrated that the phosphate adsorption performance of granular AaN-RM in a fixed-bed column was affected by various operational parameters, such as the bed depth, flow rate, initial solution pH and initial phosphate concentration. With the optimal empty-bed contact time (EBCT) of 24.27 min, the number of processed bed volumes and the phosphate adsorption capacity reached 496.95 and 84.80 mg/g, respectively. Then, the saturated fixed-bed column could be effectively regenerated with a 0.5 mol/L HCl solution. The desorption efficiency remained as high as 83.45% with a low weight loss of 3.57% in the fifth regeneration cycle. In addition, breakthrough curve modelling showed that a 5-9-1 feed-forward artificial neural network (ANN) could be effectively applied for the optimization of the fixed-bed adsorption system; the coefficient of determination (R²) and the root mean square error (RMSE) evaluated on the validation-testing data were 0.9987 and 0.0183, respectively. Therefore, granular AaN-RM fixed-bed adsorption exhibits promising potential for phosphate removal and recovery from polluted water.

Thermodynamic analysis of direct interspecies electron transfer in syntrophic methanogenesis based on the optimized energy distribution

The aim of this study was to investigate the syntrophic methanogenesis from the perspective of energy transfer and competition. Effects of redox materials and redox potential on direct interspecies electron transfer (DIET) were examined through thermodynamic analysis based on the energy distribution principle. Types of redox materials could affect the efficiency of DIET via changing the total energy supply of the syntrophic methanogenesis. Decreasing system redox potential could facilitate DIET through increasing the total available energy. The competition between hydrogenotrophic methanogens and DIET methanogens might be the reason for the low proportion of the DIET pathway in the syntrophic methanogenesis. A facilitation mechanism of DIET was proposed based on the energy distribution. Providing sufficient electrons, inhibiting hydrogenotrophic methanogens and adding more competitive redox couples to avoid hydrogen generation might be beneficial for the facilitation of DIET.

The inhibitory effect of thiosulfinate on volatile fatty acid and hydrogen production from anaerobic co-fermentation of food waste and waste activated sludge

Thiosulfinate, a nature antibiotic, existed in all parts of Allium thereby accumulating in kitchen waste vastly. However, few literatures were available related to its influence on volatile fatty acids (VFA) and hydrogen production when kitchen waste digestion technology was applied. This study aimed to explore the inhibitory effect and the relevant mechanism. Experimental results showed that the hydrogen accumulation decreased from 23.2 ± 0.8 to 8.2 ± 0.1 mL/g VSS (volatile suspended solid) and maximal total VFA yield decreased from 765.7 ± 21.2 to 376.4 ± 21.7 mg COD (chemical oxygen demand)/g VSS when the dosage of thiosulfinate increased from 0 to 12.5 µg/g VSS. The mechanism study indicated, compared with control group, that the butyric acid decreased from 59% to 20.1% of total VFA yield when reactor in present of 12.5 µg/g VSS thiosulfinate. Moreover, the relative activities of functional enzymes were inhibited 73.4% (butyryl-CoA) and 72.7% (NADH), respectively.

Effect of alkyl polyglycosides on the performance of thermophilic bacteria pretreatment for saline waste sludge hydrolysis

In this study, alkyl polyglycosides (APG) was used to further accelerate the hydrolysis of saline waste sludge with thermophilic bacteria (TB) pretreatment. In the presence of 0.4 g/g TSS APG, the concentrations of soluble chemical oxygen demand (SCOD), soluble carbohydrate and soluble protein in dissolved organic matters (DOM) were 0.4, 2.4 and 1.3 times of that without APG addition, respectively. Excitation emission matrix (EEM) fluorescence spectroscopy revealed that the addition of APG led to the increase of soluble microbial materials and the decrease of fulvic acid-like substances in DOM, which was beneficial for the subsequent process of anaerobic digestion. Using APG promoted the releasing of enzymes trapped in saline waste sludge and improved the activity of enzymes during hydrolysis. The activities of α-glucosidase and protease increased by 8.8% and 21.3% respectively in the presence of 0.4 g/g TSS APG comparing no APG addition.

Molecular evidence for the adaptive evolution of Geobacter sulfurreducens to perform dissimilatory iron reduction in natural environments

The electrically conductive pili (e-pili) of Geobacter species enable extracellular electron transfer to insoluble metallic minerals, electrodes and other microbial species, which confers biogeochemical significance and global prevalence on Geobacter in diverse anaerobic environments. E-pili are constructed by truncated PilA which is considered to have evolved from full-length pilin by gene fission under positive evolutionary selection. However, this hypothesis is based on phylogenetic analysis and has not yet been experimentally confirmed. Here, we reconstructed an ancestral strain of G. sulfurreducens (designated COMB) carrying full-length PilA by combining genes GSU1496 and GSU1497. The results demonstrated that strain COMB expressed and assembled the full-length fused PilA and exhibited an outer membrane c-type cytochrome profile similar to the wild-type strain. Surprisingly, the generated COMB-pili were also conductive, indicating the evolution of truncated PilA did not occur for conductivity. Moreover, strain COMB minimally reduced Fe(III) oxides but maintained its ability to respire electrodes, demonstrating the truncation of pilin enables iron respiration. This study provides the first experimental evidence that the truncation of pilin in Geobacter species confers adaption to Fe(III)-mineral-mediated selective pressures, and suggests an evolutionary event during which the separation of the GSU1497 gene helped Geobacter survive and thrive in natural environments.

Enhancing direct interspecies electron transfer in syntrophic-methanogenic associations with (semi)conductive iron oxides: Effects and mechanisms

Anaerobic digestion is an effective biological treatment process that produces methane by degrading organic compounds in waste/wastewater. It is a complicated microbial process by metabolic interactions among different types of microorganisms. In this process, efficient interspecies electron transfer between secondary fermenting bacteria and methanogens is the critical process for fast and effective methanogenesis. In syntrophic metabolism, hydrogen or formate has been considered as the conventional electron carrier transferring electrons from secondary fermenting bacteria to hydrogenotrophic methanogens. Recently, direct interspecies electron transfer (DIET) without the involvement of dissolved redox mediators is arousing great concerns and has been regarded as a more efficient and thermodynamically favorable interspecies electron transfer pathway for methanogenesis. Interspecies electron exchange through DIET is accomplished via the membrane-bound cytochromes or conductive pili. Several kinds of exogenously-added conductive or semiconductive iron oxides have been discovered to greatly enhance anaerobic methanogenesis through promoting DIET. Different (semi)conductive iron oxides give a boost to DIET through different mechanisms based on the physicochemical properties of the iron oxides and the reciprocal interactions between iron oxides and functional microorganisms. In this review, the current understanding of interspecies electron transfer in syntrophic-methanogenic consortions is summarized, the effects and deep-rooted mechanisms of (semi)conductive iron oxides on methanogenesis and DIET are discussed, and possible future perspectives and development directions are suggested for DIET via (semi)conductive iron oxides in anaerobic digestion.

Recyclable magnetite-enhanced electromethanogenesis for biomethane production from wastewater

Improving the yield and methane content of biogas is of great concern for wastewater treatment by anaerobic digestion. Herein we developed a nanomagnetite-enhanced electromethanogenesis (EM_nano) process for the first time, the sustainable utilization of which improved the biomethane production rate from dairy wastewater. The maximum CH₄ production rate in the EM_nano process is 2.3 ± 0.3-fold higher than it is in the conventional methanogenesis (CM) process, and it is accompanied by an almost delay-free start-up. The technical-economic evaluation revealed that an 82.1 ± 5.0% greater net benefit was obtained in the third generation of the EM_nano process compared with the CM process. The improved methanogenesis was attributed to the formation of dense planktonic cell co-aggregates that are triggered by nanomagnetite, which facilitated the interspecies electron exchange during acetoclastic methanogenesis. Simultaneously, a cathode biofilm with high viability and catalytic activity was also formed in the EM_nano process that decreased the biofilm resistance and facilitated the electron transfer during electromethanogenesis. This study is a worthwhile attempt to combine recyclable conductive materials with an electromethanogenesis process for wastewater treatment, and it effectively achieves energy recovery with high stability and cost-effectiveness.

Enhancement in the partial nitrification of wastewater sludge via low-intensity ultrasound: Effects on rapid start-up and temperature resilience

The partial nitrification process can reduce the aeration energy consumption in bioreactors by 25%. Low-intensity ultrasound (0.25 W·mL^-1) was applied during the partial nitrification process to evaluate its effects on start-up and temperature resilience. Ultrasound application led to rapid start-up of the partial nitrification process (within 18 d) with a nitrite accumulation ratio of above 80% at 18 °C. Moreover, when the temperature was increased to 28 °C, the partial nitrification process was effectively maintained with a nitrite accumulation ratio of above 80%. Ultrasonic treatment for a long duration had a positive effect on ammonia oxidizing bacteria of the genus Nitrososphaera, whereas the population of nitrite oxidizing bacteria, Nitrospira, decreased. The temperature resilience of Nitrososphaera was also enhanced. These findings indicate that ultrasound induces rapid start-up of the partial nitrification process and enhances the temperature resilience of Nitrososphaera.

The presence of ferrihydrite enhances greenhouse gas-methane emission in the environment

Aquatic system is the major source of atmospheric methane. This study explored the influences of ferrihydrite, which is widely existed in natural aquatic system, on methane emission. Results showed that the presence of ferrihydrite led to 26.4% more methane emission. By tracking the transformation of organic compounds, it is revealed that the enhanced methane emission was attributed to greater hydrolysis and degradation of refractory compounds. More specifically, the remaining humic-like substances (HS) in ferrihydrite group (46.4 mg/L-C) were only half of that in control group (80.1 g/L-C) after 30-day incubation. The X-ray photoelectron spectroscopy spectrum confirmed the more active oxidation of organics occurred in ferrihydrite group. It was also found that ferrihydrite aided in sustaining microbial activity at stationary and starvation phases. Further study on microbial communities found that ferrihydrite promoted the enrichment of both functional and electroactive genera. This study provides insights into the greenhouse gas emission in natural environment.

Disposal of Fenton sludge with anaerobic digestion and the roles of humic acids involved in Fenton sludge

Fenton sludge (FS) generated from Fenton process with high contents of iron and organic contaminants is regarded as a hazardous waste that requires to be properly disposed. Considering that Fe(III) compounds could stimulate dissimilatory iron reduction (DIR) and enrich iron reducing bacteria (IRB) that utilized Fe(III) as electron acceptor to oxidize organic matters, FS was introduced in anaerobic digestion (AD) reactors for treating wastewater meanwhile disposing FS. Results showed that methane production and organic matters removal significantly increased with dosing FS from 0 to 2.4 g. Also, a majority of organic matters involved in FS was mineralized, including 38.5% of PAHs removal. Humic acids (HA) with redox-activity involved in FS might affect efficiency of DIR. After extracting HA from FS, the rate and the extent of Fe(III) reduction of FS decreased by 33.2% and 13.9%, respectively. Together with analysis of the electron exchange capacity of HA, it suggested that the HA involved in FS might serve as an electron shuttle to effectively promote DIR. The increase of sludge conductivity and the enrichment of IRBs in microbial communities with dosage of FS were in agreement with the above results.