Response of syntrophic aggregates to the magnetite loss in continuous anaerobic bioreactor

Electrical stress and acid orange 7 synergistically clear the blockage of electron flow in the methanogenesis of low-strength wastewater

Energy recovery from low-strength wastewater through anaerobic methanogenesis is constrained by limited substrate availability. The development of efficient methanogenic communities is critical but challenging. Here we develop a strategy to acclimate methanogenic communities using conductive carrier (CC), electrical stress (ES), and Acid Orange 7 (AO7) in a modified biofilter. The synergistic integration of CC, ES, and AO7 precipitated a remarkable 72-fold surge in methane production rate compared to the baseline. This increase was attributed to an altered methanogenic community function, independent of the continuous presence of AO7 and ES. AO7 acted as an external electron acceptor, accelerating acetogenesis from fermentation intermediates, restructuring the bacterial community, and enriching electroactive bacteria (EAB). Meanwhile, CC and ES orchestrated the assembly of the archaeal community and promoted electrotrophic methanogens, enhancing acetotrophic methanogenesis electron flow via a mechanism distinct from direct electrochemical interactions. The collective application of CC, ES, and AO7 effectively mitigated electron flow impediments in low-strength wastewater methanogenesis, achieving an additional 34% electron recovery from the substrate. This study proposes a new method of amending anaerobic digestion systems with conductive materials to advance wastewater treatment, sustainability, and energy self-sufficiency.

Magnetite alleviating calcification of anaerobic granular sludge (AnGS): Electron transfer enhancement and ion competition

Calcification accompanied by deactivation of anaerobic granular sludge (AnGS) is a continuing challenge for high calcium wastewater treatment. The interaction between Ca2+ and extracellular polymeric substances (EPS) is a precondition for this problem. In this study, magnetite for activity recovery and calcification alleviation simultaneously of AnGS under high calcium stress was investigated. The results showed that, in the presence of magnetite, the relative biogas production increased by 13.2 % with the higher activities of key enzymes involved in methanogenesis. Methanosarcina turned into the dominant methanogens, and syntrophic bacteria such as Chloroflexi, Synergistota were enriched, which indicated the enhancement of electron transfer by magnetite, supported by an 18 % increase of the electron transfer system (ETS) activity. Further characterizations of AnGS suggested that the granule calcification was alleviated with a final decrease of 13-40 % calcium content of AnGS with particle size of 1-2.5 mm. Besides, calcium was partially substituted by iron in the EPS, and the secretion of EPS especially proteins decreased. Batch tests demonstrated the competition between Fe2+ dissolved from magnetite and Ca2+, which interfered the interaction between Ca2+ and EPS, so the granule calcification was prevented. Therefore, magnetite played a pluripotent role in the alleviation of granule calcification and deactivation in situ via (1) enhancing electron transfer, and (2) blocking the complex between Ca2+ and EPS. This study provides a novel insight into the application of conductive metal materials in biological wastewater treatment systems suffering from high calcium attack.

Regulation of extracellular polymers based on quorum sensing in wastewater biological treatment from mechanisms to applications: A critical review

Extracellular polymeric substances (EPS) regulated by quorum sensing (QS) could directly mediate adhesion between microorganisms and form tight microbial aggregates. Besides, EPS have redox properties, which can facilitate electron transfer for promoting electroactive bacteria. Currently, the applications research on improving wastewater biological treatment performance based on QS regulated EPS have been widely reported, but reviews on the level of QS regulated EPS to enhance EPS function in microbial systems are still lacking. This work proposes the potential mechanisms of EPS synthesis by QS regulation from the viewpoint of material metabolism and energy metabolism, and summarizes the effects of QS on EPS synthesis. By synthesizing the role of QS in EPS regulation, we further point out the applications of QS-regulated EPS in wastewater biological treatment, which involve a series of aspects such as strengthening microbial colonization, mitigating membrane biofouling, improving the shock resistance of microbial metabolic systems, and strengthening the electron transfer capacity of microbial metabolic systems. According to this comprehensive review, future research on QS-regulated EPS should focus on the exploration of the micro-mechanisms, and economic regulation strategies for QS-regulated EPS should be developed, while the stability of QS-regulated EPS in long-term production experimental research should be further demonstrated.

Deciphering anaerobic ethanol metabolic pathways shaped by operational modes

Efficient anaerobic digestion requires the syntrophic cooperation among diverse microorganisms with various metabolic pathways. In this study, two operational modes, i.e., the sequencing batch reactor (SBR) and the continuous-flow reactor (CFR), were adopted in ethanol-fed systems with or without the supplement of powdered activated carbon (PAC) to examine their effects on ethanol metabolic pathways. Notably, the operational mode of SBR and the presence of CO2 facilitated ethanol metabolism towards propionate production. This was further evidenced by the dominance of Desulfobulbus, and the increased relative abundances of enzymes (EC: 1.2.7.1 and 1.2.7.11) involved in CO2 metabolism in SBRs. Moreover, SBRs exhibited superior biomass-based rates of ethanol degradation and methanogenesis, surpassing those in CFRs by 53.1% and 22.3%, respectively. Remarkably, CFRs with the extended solids retention time enriched high relative abundances of Geobacter of 71.7% and 70.4% under conditions with and without the addition of PAC, respectively. Although both long-term and short-term PAC additions led to the increased sludge conductivity and a reduced methanogenic lag phase, only the long-term PAC addition resulted in enhanced rates of ethanol degradation and propionate production/degradation. The strategies by adjusting operational mode and PAC addition could be adopted for modulating the anaerobic ethanol metabolic pathway and enriching Geobacter.

Magnetite-boosted syntrophic conversion of acetate to methane during thermophilic anaerobic digestion

Using a batch thermophilic anaerobic system established with 60 mL serum bottles, the mechanism on how microbial enrichments obtained from magnetite-amended paddy soil via repeated batch cultivation affected methane production from acetate was investigated. Magnetite-amended enrichments (MAEs) can improve the methane production rate rather than the methane yield. Compared with magnetite-unamended enrichments, the methane production rate in MAE was improved by 50%, concomitant with the pronounced electrochemical response, high electron transfer capacity, and fast acetate degradation. The promoting effects might be ascribed to direct interspecies electron transfer facilitated by magnetite, where magnetite might function as electron conduits to link the acetate oxidizers (Anaerolineaceae and Peptococcaceae) with methanogens (Methanosarcinaceae). The findings demonstrated the potential application of MAE for boosting methanogenic performance during thermophilic anaerobic digestion.

Functional biochar in enhanced anaerobic digestion: Synthesis, performances, and mechanisms

Anaerobic digestion technology is crucial in bioenergy recovery and organic waste management. At the same time, it often encounters challenges such as low organic digestibility and inhibition of toxic substances, resulting in low biomethane yields. Biochar has recently been used in anaerobic digestion to alleviate toxicity inhibition, improve the stability of anaerobic digestion processes, and increase methane yields. However, the practical application of biochar is limited, for the properties of pristine biochar significantly affect its application in anaerobic digestion. Although much research focuses on understanding original biochar's fundamental properties and functionalization, there are few reviews on the applications of functional biochar and the effects of critical properties of pristine biochar on anaerobic digestion. This review systematically reviewed functionalization strategies, key performances, and applications of functional biochar in anaerobic digestion. The properties determining the role of biochar were reviewed, the synthesis methods of functional biochar were summarized and compared, the mechanism of functional biochar was discussed, and the factors affecting the function of functional biochar were reviewed. This review provided a comprehensive understanding of functional biochar in anaerobic digestion processes, which would be helpful for the development and applications of engineered biochar.

Mechanisms for enhanced lignin humification with reduced organic matter loss by goethite in biogas residue composting

In this study, effects of three iron (oxyhydr)oxides on the biogas residue composting, i.e., composting with goethite (CFe1), hematite (CFe2) or magnetite (CFe3), were investigated. Results showed that composting performance of CFe1 was much better than those of CFe2 and CFe3. Addition of goethite increased temperature of CFe1 and enhanced lignin humification. More than 31.49% of Fe(III) in goethite was reduced to amorphous Fe(II) during the composting, suggesting that goethite worked as electron acceptor for microbial metabolism and heat generation. The functional bacteria Chloroflexi and Actinobacteria, and genes encoding key enzymes (AA1 family), which play essential roles in humification of lignin, were enriched in CFe1. Besides, goethite reduced 10.96% organic matter (OM) loss probably by increasing the molecular size and aggregation of OM for its protection during the composting. This study shows that adding goethite is an efficient strategy to enhancing the humification of lignin-rich biowaste.

Enhancement of methane production in anaerobic digestion of high salinity organic wastewater: The synergistic effect of nano-magnetite and potassium ions

During high salinity organic wastewater (HSOW) anaerobic digestion treatment, the process of methanogenesis can be severely inhibited in the high salinity environment, and the accumulation of volatile organic acids (VFAs) leads to failure of the anaerobic reaction. In this study, nano-magnetite and KCl were adopted to alleviate the inhibitory effect of high salinity and enhance the HSOW anaerobic digestion performance. The result showed that, under the optimal dosage of 200 mg/L, nano-magnetite addition promoted the anaerobic digestion performance, and the methane production increased by 11.06%. When KCl was added with a dosage of 0.174%, the methane production increased by 98.37%. The simultaneous addition of nano-magnetite (200 mg/L) and KCl showed a synergistic effect on enhancing HSOW anaerobic digestion performance, and the methane production increased by 124.85%. The addition of nano-magnetite and KCl promoted the conversion of VFAs, especially accelerated the degradation of propionic acid and butyric acid, also it promoted the activity of acetate kinase, dehydrogenase and F₄₂₀, and thereby enhanced the methanogenesis process. This study could provide a new method for enhancing the anaerobic digestion of HSOW.

Role of extracellular polymeric substances in methane production from waste activated sludge induced by conductive materials

Conductive materials have been widely used to establish direct interspecies electron transfer (DIET) for enhancing methane production potential from anaerobic digestion of waste activated sludge (WAS). However, the roles of extracellular polymeric substances (EPSs) affected by conductive materials on anaerobic digestion have been rarely reported. This study selected four widely used conductive materials, i.e., granular active carbon (GAC), biochar (BC), zero-valent iron (ZVI), and magnetite (Mag), to reveal the roles of EPSs. Results showed that methane production potentials were increased by BC, ZVI and Mag compared to that of control, with increase ratios of 13.4 %, 22.2 %, and 12.2 %, while a decrease was observed by GAC. The contents, components and characteristics of EPSs were all affected by conductive materials. The contents of EPSs were increased by ZVI and Mag, while they were decreased by BC and GAC. The ratios between proteins and polysaccharides (PN/PS) in loosely bound EPSs (LB-EPSs) were reduced in all groups, while they were similar in tightly bound EPSs (TB-EPSs) of ZVI and Mag groups. In addition, the cytochrome C and redox properties were remarkably promoted in suspension rather than in LB- and TB-EPSs. It was found that the correlation relationships between the maximal methane production potential (Pmax) and PN/PS in EPSs were positive, as well as fluorescent substances, especially tyrosine-like and tryptophan-like substances, with R² of 0.96 and 0.98. Furthermore, the correlation relationships also existed between EPSs and microbial communities. Clostridium and Methanobacterium, potential DIET partners, presented significant positive correlation relationships (P < 0.05) with Pmax, PN/PS and fluorescent substances in EPSs. The findings may provide some new insights for mechanism investigation of anaerobic digestion induced by conductive materials.

Effects of submicron magnetite particles on granulation of flocculent sludge and process stability in upflow anaerobic sludge blanket reactor

Promoting direct interspecies electron transfer (DIET) with conductive additives is considered a promising approach to enhance methanogenesis. This study investigated the effects of adding submicron magnetite particles on sludge granulation and methanogenic performance in upflow anaerobic sludge blanket reactors inoculated with flocculent sludge. The reactor supplemented with magnetite was more stable and resilient than the no-magnetite control, with higher degree of granulation (up to 26.6-fold) and biomass retention. Magnetite addition to unstable reactors improved the methane yield in both reactors (1.2-1.3-fold). Electroactive Deltaproteobacteria bacteria, including Geobacter and Syntrophobacter, were enriched in the presence of magnetite. Methanogenic functional genes involved in DIET-based syntrophy were more abundant under magnetite-supplemented conditions. However, the improvement of methanogenic performance and granulation was limited, and inducing the self-embedment of magnetite into mature sludge granules rather than granulating flocculent sludge with magnetite appears to be a better strategy for engineering DIET in anaerobic granular sludge systems.

Chemical structure and nanomechanics relevant electrochemistry of solid-phase humic acid along a typical forest-river-paddy landscape section in eastern China and its environmental implications

Solid-phase humic acid (HA_solid) plays a critical role in global carbon cycle and redox biogeochemistry of topsoil, because of its unique physicochemical properties, including electrochemical property. In this study, topsoil HA_solid along a typical forest-river-paddy landscape section in eastern China was investigated in the aspects of electrochemical property, chemical structure and nanomechanics, and their relationship. Nano-size HA_solid particles were extracted from topsoil of paddy soil (PS-HA_solid), forest soil (FS-HA_solid) and riverside sediment (RS-HA_solid). Results showed that all the HA_solid were conductive and played an important role in conductivity of topsoil suspension. HA_solid contained both reversible and irreversible redox peaks, with redox activity of PS-HA_solid > RS-HA_solid > FS-HA_solid. Owing to limited humification, electron exchange capacity (EEC) values of topsoil HA_solid suspensions were modest (3.18-4.45 μmol e^-g^-1 HA_solid). Compared with RS-HA_solid and FS-HA_solid, PS-HA_solid showed higher aromaticity and higher degree of humification with simple and even nanomechanical property. Long-term cultivation (human activities) as well as high content of polyvalent metals in paddy soil were supposed to facilitate formation of aromatic carbon and improve humification of HA_solid. The results suggested that aromatic carbon and high humification degree of PS-HA_solid contributed to simple and even nanomechanics, which further optimized its electrochemical property. This study not only provides novel insight into the mechanism of HA_solid mediating electron transfer, but also inspires ideas for soil and environmental management with different purposes based on regulation of HA_solid.

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.

Novel insight on ferric ions addition to mitigate recalcitrant formation during thermal-alkali hydrolysis to enhance biomethanation

Thermal-chemical pre-treatment has proven to facilitate the solubilization of organics and improvement in biogas generation from the organic fraction of municipal solid waste (OFMSW). However, the production of recalcitrant is inevitable when OFMSW is pretreated at high temperatures and alkali dosage. This study develops a strategy to use Fe³⁺ to reduce the formation of recalcitrant compounds, i.e., 5-HydroxyMethyl Furfural (5-HMF), furfurals, and humic acids (HA) during thermal-alkali pre-treatment. It was postulated that the formation of the recalcitrant compound during pre-treatment can be reduced by Fe³⁺ dosing to oxidize intermediates of Maillard reactions. A decrease in 5-HMF (45-49%) and furfurals (54-66%) was observed during Fe³⁺ (optimum dose: 10 mg/L) mediated thermal-alkali pre-treatment owing to the Lewis acid behavior of FeCl₃. The Fe³⁺ mediated assays show a substantial improvement in VS removal (28%) and biogas yield, i.e., 31% (292 mL/gVS_added) in 150 °C + 3 g/L NaOH, 34% (316 mL/gVS_added) in 175 °C + 3 g/L NaOH, and 36% (205 mL/gVS_added) in 200 °C + 3 g/L NaOH assays, over their respective controls (no Fe³⁺ dosing). The reducing property of Fe³⁺ rendered a low ORP (-345 mV) in the system than control, which is beneficial to the anaerobic microbiome. Electrical conductivity (EC) also shows a three-fold increase in Fe³⁺ mediated assays over control, promoting direct interspecies electron transfer (DIET) amongst microbes involved in the electrical syntrophy. The score plot and loading plots from principal component analysis (PCA) showed that the results obtained by supplementing 10 mg/L Fe³⁺ at 150, 175, and 200 °C were significantly different. The correlation of the operational parameters was also mutually correlated. This work provides a techno-economically and environmentally feasible option to mitigate the formation of recalcitrant compounds and enhance biogas production in downstream AD by improving the degradability of pretreated substrate.

Role and significance of water and acid washing on biochar for regulating methane production from waste activated sludge

Methane recovered from anaerobic digestion of waste activated sludge (WAS) can be used as the energy supplement of the wastewater treatment plant, benefiting to its carbon-neutral operation. In order to enhance methane production, biochar (BC) has been widely selected as conductive material to build direct interspecies electron transfer (DIET) in anaerobic digestion of WAS. However, the role and significance of washing strategies, including water and acid washing, on BCs for regulating methane production have not been reported. This study selected the frequently used woody- (W) and straw (S)-BCs as mode. Compared to raw W-BC, water and acid washing W-BC increased the methane yields by 19.1% and 15.7%, respectively. Differently, the methane yields among raw, water and acid washing S-BCs were similar. Mechanism study showed that both the two washing strategies optimized the properties of raw W-BC for promoting methane production. Water and acid washing W-BCs increased the electron transfer functional groups, such as ketones and quinones, which were not observed in S-BCs. Moreover, the electron-active microorganisms were enriched with the presence of water and acid washing W-BCs, and the predominant pathway for methane production shifted from hydrogentrophic to acetotrophic and DIET methanogenesis, while the microbial communities, including bacteria and archaea, were similar with the presence of raw, water and acid washing S-BCs. These findings of this work provide some new insights for production improvement regulation of methane from anaerobic digestion of wastes induced by BCs.

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.

Independent of direct interspecies electron transfer: Magnetite-mediated sulphur cycle for anaerobic degradation of benzoate under low-concentration sulphate conditions

The aim of this study was to investigate the primary mechanism of magnetite promoting anaerobic degradation of aromatic compounds under the low-concentration sulphate conditions. Under influent conditions of benzoate at 50 mM-chemical oxygen demand (COD) and sulphate at 15 mM, magnetite promoted benzoate degradation (77.1% vs 56.3%), while the effluent sulphate concentration was slightly higher than that without magnetite (1.6 mM vs 0.7 mM), inconsistent with functional gene prediction that both sulphate respiration and sulphur compound respiration were relatively more active in the presence of magnetite. Remarkably, X-ray diffraction showed that, signal related to Fe₃O₄ faded away and finally was replaced by FeSO₄ and FeS, indicating that magnetite participated in benzoate degradation coupled to sulphate reduction via dissimilatory Fe(III) reduction. Further X-ray photoelectron spectroscopy showed that, signal related to S⁰ was only detected with magnetite, suggesting the possibility of re-oxidation of sulphide to elemental sulphur coupled to Fe(III) reduction. This was further supported by the increase in abundance of Desulfuromonas acetexigens capable of growing on Fe(III). In addition, magnetite specially enriched the chemolithotrophic sulphur-disproportionating microbes, Desulfovibrio aminophilus, which might proceed the disproportionation of elemental sulphur to sulphate and sulphide to achieve a sulphur cycle for benzoate degradation.

Formation and characterization of conductive magnetite-embedded granules in upflow anaerobic sludge blanket reactor treating dairy wastewater

Promoting direct interspecies electron transfer (DIET) with conductive additives has proved effective in improving anaerobic digestion performance and stability. However, its application is limited by the need to replenish the washout loss of conductive materials. This study reports the formation of conductive magnetite-embedded granular sludge and its long-term influence on the performance of upflow anaerobic sludge blanket reactors treating dairy wastewater. The magnetite-supplemented reactor maintained better performance than the no-magnetite control, with greater sludge settling and electron transport activity, throughout the 192-d experiment at increasing organic loading rates (1.2-8.5 g chemical oxygen demand/L·d). The abundance of electroactive microbes also remained higher in the magnetite-supplemented reactor. The results suggest that DIET-based electric syntrophy was promoted in the magnetite-embedded granules. This study is the first to demonstrate the self-embedment of submicron conductive material into granular sludge and its benefits. These findings offer a new approach to enhancing anaerobic granular sludge systems.

The effects of magnetite on anammox performance: Phenomena to mechanisms

Low temperature is adverse to anaerobic ammonium oxidation (anammox) reaction while proper Fe addition can enhance anammox performance. Therefore, batch assays were conducted to investigate the potential effects of magnetite (100 μm, 20 μm and 200 nm) on anammox performance which were achieved from the reactor operated at 10-25 °C. After 3 runs, the results indicated that nano-scale magnetite improved the nitrogen elimination significantly. The specific anammox activity (SAA) of the group with nano-magnetite amendments was greater than the other groups after 3 runs (13.5, 12.9, 14.3, 15.4 and 15.7 mgTN/(gVSS·h)), reaching 18.0 mgTN/(gVSS·h). The distribution of magnetite in the granules were then analyzed using X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The results indicated that nano-magnetite was more feasible to attached to the surface of the granules which might accelerate the release of Fe(II) or Fe(III) to enhance anammox performance.

Magnetite drives self-dechlorination of 4-chlorophenol in anoxic aquatic sediments

The lack of available electron donors is well known as a major factor limiting the efficiency of microbial dechlorination of 4-chlorophenol (4-CP) in anoxic aquatic sediments. Considering that Fe(III) minerals largely contained in sediments can especially enrich Fe(III)-reducing bacteria and unlock the ring-like intermediates produced by dechlorination of 4-CP via dissimilatory Fe(III) reduction, a strategy of self-dechlorination of 4-CP utilizing its metabolism intermediates such as short-fatty acids (SCFAs) as the endogenous electron donors with magnetite was proposed in this study. The results showed that the removal efficiency of 4-CP increased by 156-203% in magnetite-supplemented biotic groups compared with the magnetite-free biotic group. Liquid chromatography-mass spectrometer (LC-MS) and gas chromatography (GC) revealed the possible metabolic pathway of anoxic 4-CP degradation with magnetite: 4-CP→phenol→cyclohexene-1-carboxylic acid→2-hydroxycyclohexanecarboxylic acid→hexanoic acid/valeric acid→butyric/propionic acids→CO₂. High-throughput sequencing analysis showed that the abundance of functional bacteria, Desulfuromonas, Pseudomonas and Bacillus species, were increased by 1.38-1.97, 1.50-2.04, and 11.60-17.18 folds in magnetite-supplemented biotic groups, compared with the magnetite-free biotic groups. Analysis of Fe²⁺ concentration and cyclic voltammetry (CV) suggested that the potential Fe(III)/Fe(II) transformation occurred and proceeded the anoxic 4-CP degradation continuously.

Insight into the performance and microbial community profiles of magnetite-amended anaerobic digestion: Varying promotion effects at increased loads

In current study, the enhancement effect of magnetite on anaerobic digestion was evaluated at increased organic loading rate (OLR) from 1.6 to 25.6 kg COD·m^-3·d^-1. The supplement of magnetite enhanced the methane yield by 7-483% accompanied with faster VFAs conversion. Microbial analysis suggested the varied enhancing effect achieved at different OLRs was attributed to different syntrophic interactions triggered by magnetite. More specially, an electroactive syntropy was established between Trichococcus with Methanobacterium at OLR lower than 6.4 kg COD·m^-3·d^-1, while with the OLR increase, more acid fermentative bacteria (Propionimicrobium, Syner-01) were enriched and further enhanced methanogenesis in a syntrophic way with Methanosaeta. Overall, the incorporation of magnetite was a promising approach to achieve efficient anaerobic digestion, OLR was also critical factor affecting the methanogenesis and should be carefully regulated in future application.

Enhancing anaerobic digestion of kitchen wastes via combining ethanol-type fermentation with magnetite: Potential for stimulating secretion of extracellular polymeric substances

Magnetite, a common mineral that is abundant in the soils and sediments, has been widely documented to enhance the anaerobic digestion of organic wastes, whereas the mechanisms of magnetite promoting interspecies electron transfer are still unclear. In this study, under the conditions (ethanol-type fermentation) employed, magnetite stimulated the secretion of extracellular polymeric substances (EPS). Analysis of three-dimensional excitation emission matrix revealed that these EPS secreted in the presence of magnetite were primarily comprised of the redox-active organic functional groups. Electrochemical analysis showed that the EPS secreted with magnetite had the higher electron-accepting and electron-donating capacity than the EPS without magnetite. Syntrophomonas species capable of extracellularly transferring electron were enriched with supplementing magnetite. Together with the increased abundance of Methanospirillum and Methanobacterium species that could proceed direct interspecies electron transfer (DIET), the anaerobic digestion was likely improved due to the establishment of DIET with supplementing magnetite. As a result, anaerobic digestion of kitchen wastes was evidently enhanced. With decreasing the solid retention time to 30 d, the methane production rate only slightly declined to 18 ± 0.8 mL/g-VSS/d in the magnetite-supplemented digester, while almost no methane was detected in the digester without magnetite.

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.

Enhanced production of methane in anaerobic water treatment as mediated by the immobilized fungi

Anaerobic digestion of organic waste and wastewater represents an attractive sustainable bio-technology to produce methane as an alternative to fossil energy. In response to improvement of methane production via enhancing methanogenesis, current strategies of the addition of external biological/non-biological materials have to confront either the loss of materials, high cost and/or possible destruction of the microbial community. Here, we report the first case of using immobilized fungi Aspergillus sydowii 8L-9-F02 to optimize the microbial community, achieving remarkable improvement of the methane production in both batch test (1.5 times) and continuous flow operation (1.13-1.31 times). The crucial role of fungi is associated with the stimulation of enrichment of Methanosaeta and Methanobacterium for methanogenesis from 28.2 to 67.4% as well as the improved activity of enzyme F420. Moreover, fungi also increase the content of extracellular polymeric substances, facilitating the formation of bio-aggregates. This work provides a new pathway to enhance methanogenesis during anaerobic digestion of wastewater by using fungi as bio-enhancer.

Accelerated anaerobic biodecolorization of sulfonated azo dyes by magnetite nanoparticles as potential electron transfer mediators

Anaerobic decolorization of azo dyes has been evidenced to be an economical and effective pretreatment method, but its generally limited by the low decolorization efficiency, especially for biodecolorization sulfonated azo dyes. In this study, magnetite nanoparticles (MNPs) as a conductive material, was coupled into anaerobic system for enhancing decolorization of sulfonated azo dyes, i.e., methyl orange (MO), with technology feasibility and system stability emphasized. The results showed that the anaerobic decolorization capacity was significantly enhanced with addition of MNPs (at dose of 1 g/L), where the efficiencies of MO decolorization and aromatic amines formation were as high as 97.28 ± 0.78 % and 99.44 ± 0.25%, respectively. In addition, both electron transport system activity and sludge conductivity were also significantly improved, suggesting that a direct extracellular electron transfer had been successfully established via MNPs as RMs. Under continuous-flow experiments, addition of MNPs not only improved anaerobic system resistance environmental stress (e.g., high MO concentration, low hydraulic retention time and low co-substance concentration) but also accelerated sludge granulation. The relative abundance of functional species related to dissimilatory iron reduction and MO biodegradation were also enriched under MNPs stimulation. The observed long-term stable performance suggests the full-scale application potential of this coupled system for treatment of wastewater containing sulfonated azo dyes.

Biochar facilitates rapid restoration of methanogenesis by enhancing direct interspecies electron transfer after high organic loading shock

This study evaluated the effectiveness of biochar addition against high organic loading shock (OLS) of 80 kg COD/m³/d in up-flow anaerobic sludge blanket (UASB) reactors (R1 with biochar; R2 without biochar). After OLS of 24 h, R2 suffered the irreversible acidification (pH of 5.42 ± 0.07) with low biogas production of 0.08 ± 0.01 m³/kg COD/d. In contrast, the biogas production in R1 restored rapidly to 0.33 ± 0.04 m³/kg COD/d, and effluent pH in R1 returned to 7.01 ± 0.22. With addition of biochar, potential direct interspecies electron transfer (DIET) partners, including volatile fatty acids (VFAs)-oxidizing bacteria (Bacteroidetes, Smithella, Desulfovibrio, Geobacter) and methanogens (Methanosaeta, Methanosarcina) were enriched in R1, which were conductive to maintain the balance of acidogenesis and methanogenesis. Moreover, the retention of Methanosaeta and Methanosarcina coupled with biochar maintained the structural stability of granular sludge in R1 under the pressure of OLS and VFAs, which guaranteed the stability of anaerobic system.

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.

Role of magnetite in methanogenic degradation of different substances

For better understanding the role of magnetite in methanation of different substrates, two up-flow anaerobic sludge bed reactors (RM with magnetite; RS with silica) were built using acetate (stage I), propionate + butyrate (stage II), and sucrose (stage III) as the substrates, respectively. RM reactor showed better COD removal efficiency and adaptability to different substrate impacts. More extracellular polymeric substances (EPS) were produced for anaerobic sludge granulation, and the sludge in RM had better intensity, hydrophobicity and electroactivity compared with those in RS. Interestingly, magnetite had different promoting effects on methanogenic degradation of different substrates, and magnetite facilitated different syntrophic partners, like Desulfovibrio, Smithella, unidentified Clostridiates and Methanosaeta in different stages. The strengthening factor of biogas production from sucrose was the highest (1.23 ± 0.03), and analysis of key enzyme activities indicated that the potential magnetite-induced direct interspecies electron transfer (DIET) improved the process between the glycolysis, oxidation of pyruvate and CO₂-dependent methanogenesis.

Simultaneous nitrate, nickel ions and phosphorus removal in a bioreactor containing a novel composite material

This study presents the novel composite material TMCC/PAA/SA@Fe(TPSA), a bacteria immobilized carrier for use in bioreactor systems to enhance the simultaneous removal efficiency of nitrate, Ni(II) and phosphorus. The influence of various operational factors were evaluated on the performance of nitrate, phosphorus and Ni(II) removal. Results demonstrate that under optimum conditions of an hydraulic retention time (HRT) of 8 h and pH 7.0, nitrate and phosphorus removal reached nearly 100% and 61.7%, respectively. When the initial Ni(II) concentration was 1 mg/L, approximately 100% Ni(II) removal efficiency was achieved. Furthermore, the morphology and components of the TPSA immobilized bacterial pellets were analyzed to investigate the mechanism of simultaneous nitrate, Ni(II) and phosphorus removal. Microbial metabolism was more active in the experimental reactor compared with control, although high concentrations of Ni(II) could inhibit bacterial activity.