Ferroferric oxide triggered possible direct interspecies electron transfer between Syntrophomonas and Methanosaeta to enhance waste activated sludge anaerobic digestion

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.

Effects of foam nickel supplementation on anaerobic digestion: Direct interspecies electron transfer

Stimulating direct interspecies electron transfer with conductive materials is a promising strategy to overcome the limitation of electron transfer efficiency in syntrophic methanogenesis of industrial wastewater. This paper assessed the impact of conductive foam nickel (FN) supplementation on syntrophic methanogenesis and found that addition of 2.45 g/L FN in anaerobic digestion increased the maximum methane production rate by 27.4 % (on day 3) while decreasing the peak production time by 33 % as compared to the control with no FN. Cumulative methane production from day 2 to 6 was 14.5 % higher with addition of 2.45 g/L FN than in the control. Levels of FN in excess of 2.45 g/L did not show benefits. Cyclic voltammetry results indicated that the biofilm formed on the FN could generate electrons. The dominant bacterial genera in suspended sludge were Dechlorobacter and Rikenellaceae DMER64, whereas that in the FN biofilm was Clostridium sensu stricto 11. The dominant archaea Methanosaeta in the FN biofilm was enriched by 14.1 % as compared to the control.

Nano iron materials enhance food waste fermentation

Anaerobic fermentation can produce valuable chemicals from food waste, such as butyric acid, lactic acid, and ethanol. Three nano iron materials, i.e. zero-valence iron (nZVI), ferric oxide (nFO), and magnetite (nM), were tested to improve food waste fermentation with mixed inoculation. nZVI and nFO enhanced the dissolution and conversion of carbohydrate to lactate, dependent on different mechanisms, while nM had almost no effects due to its aggregation at high doses. nZVI alleviated the drop of pH, kept the ORP at an anaerobic level, increased functional enzymes, and resulted in a high hydrolysis rate of 45% and lactic acid yield of 0.16 g/g VS_fed at the dose of 500 mg/(L_fed·d). nFO increased the ORP to 100 mV, stimulated the enrichment of Pseudomonas, and increased the abundance of Lactobacillus_panis, leading to hydrolysis rate of 47% and lactic acid yield of 0.19 g/gVS_fed at the dose of 200 mg/(L_fed·d).

Fe3O4 accelerates tetracycline degradation during anaerobic digestion: Synergistic role of adsorption and microbial metabolism

Antibiotics contaminants, for example, tetracycline (TC) in the environment have attracted extensive attention around the world, and appropriate treatments for such contaminants are urgently required. In this study, five groups of anaerobic reactors supplemented with different amounts of Fe₃O₄ were operated periodically to investigate their performance on TC removal. The results showed that Fe₃O₄ effectively promoted TC removal. Compared with the control reactor, the TC removal efficiency was increased by 7.3% when co-digested with glucose, and increased by 40.4% when mono TC was digested in reactors with 5.0 g/L Fe₃O₄. Further analysis indicated that the probable mechanism of Fe₃O₄ promoting TC removal was through TC being adsorbed from the liquid onto Fe₃O₄, making TC more available for microbes to be biodegraded. Microbial community analysis indicated that the bacteria (Klebsiella, Pseudomonas, and Escherichia) related to TC removal were enriched, which meant more pathways for TC removal were available following the addition of Fe₃O₄. In addition, in the Fe₃O₄-supplemented reactors, syntrophic metabolism (between Desulfovibrio and Methanobacterium, Azonexus and Methanobacterium) were possibly established, which played an important role in improving TC removal and CH₄ production. The electron transport system data further confirmed these results. The functional gene classification for Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis demonstrated that the dominant functions enhanced by Fe₃O₄ supplementation was microbial metabolic activities.

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

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

Novel strategy for relieving acid accumulation by enriching syntrophic associations of syntrophic fatty acid-oxidation bacteria and H2/formate-scavenging methanogens in anaerobic digestion

Aiming at relieving acid accumulation in anaerobic digestion (AD), syntrophic associations of syntrophic fatty acid-oxidation bacteria and H₂/formate-scavenging methanogens were enriched by feeding propionate, butyrate and formate in an up-flow anaerobic sludge blanket (UASB) reactor. Results showed that methane yield increased by 50% with increasing formate concentration (0-2000 mg COD/L). In addition, the abundance and quantity of SFOB (Syntrophobacter, Smithella and Syntrophomonas) and H₂/formate-scavenging methanogens (Methanobacteriales and Methanomicrobiales) were increased after microbial acclimation. The enriched syntrophic associations showed higher propionate and butyrate removal efficiencies of 98.48 ± 1.14% and 99.71 ± 0.71%, respectively. Furthermore, encoding genes of formate dehydrogenase and hydrogenases presented higher abundances after microbial enrichment, which suggested that the enhancements of interspecies formate transfer and interspecies hydrogen transfer between syntrophic associations benefited volatile fatty acids (VFAs) conversion. This research provided an effective strategy to relieve acid accumulation.

Improving CH4 production and energy conversion from CO2 and H2 feedstock gases with mixed methanogenic community over Fe nanoparticles

To achieve methanogenic community optimization and improve the conversion efficiency of CO₂ to CH₄, Fe nanoparticles were used to promote the Methanothermobacter abundance in methanogens, which significantly increased the conversion efficiency of CO₂ and H₂ feedstock gases to CH₄ product. High-throughput 16S rRNA gene sequencing analysis revealed that Methanothermobacter abundance markedly increased from 7 to 16% when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R (the working volume in the bioreactor). Therefore, the CH₄ yield significantly promoted from 0.105 to 0.186 L/L_R. However, when the Fe nanoparticles concentration was further increased to 2 g/L_R, methanogenesis was inhibited due to toxic effects. The electron transfer constant k_app of anaerobic sludge increased by 32.8-fold to 5.77 × 10^-2 s^-1 when the Fe nanoparticles concentration increased from 0 to 1.5 g/L_R, which significantly promoted carbon conversion efficiency from 52.9 to 92.9% and energy conversion efficiency from 46.3 to 76.9%.

Effect of steel slag in recycling waste activated sludge to produce anaerobic granular sludge

The amount of waste activated sludge (WAS) has grown dramatically in China. WAS is considered as a problematic and hazardous waste, which should be disposed in a safe and sustainable manner. In order to recycle WAS to an anaerobic granular sludge (AnGS) process for anaerobic digestion, Fe powder and steel slags (rusty and clean slags) were used to enhance the granulation process. The results demonstrated that both rusty and clean slags encouraged the development of granular sludge. Adding 10 g/L clean slags could increase AnGS granulation rate by 37%. In the presence of clean slags, extracellular polymeric substances (EPS) concentration in granules increased noticeably to 715 mg/g mixed liquor suspended solids (MLSS). High throughput sequencing analysis exhibited more diversity and higher abundance of functional microbial communities in the batch bottle with 10 g/L clean slags. This study suggested that adding clean slags at 10 g/L dosage was a sustainable and effective method for the sludge granulation.

Improved anaerobic digestion efficiency of high-solid sewage sludge by enhanced direct interspecies electron transfer with activated carbon mediator

High-solid anaerobic digestion (AD) faces the problems of easy acidification and low methane production efficiency. In this study, activated carbon (AC)-enhanced direct interspecies electron transfer (DIET) was investigated to overcome such problems. Results showed the conversion of volatile fatty acids (VFAs) into methane rate was increased with AC addition, which led improved methane production efficiency. The methane yields from the early AD stage improved by 124.0-146.3% with AC addition. The T₈₀ shortened by 8-9 days with AC addition. The relative abundances of Geobacter, Syntrophomonas and Methanosaeta that associated with DIET improved for 63.65%, 256.3% and 4.35% by AC addition, which reflected the enhanced DIET with AC addition. The redox activity of AC might be responsible for the enhanced DIET. This study would advance the understanding of DIET and provide a potential solution to the problems existed in high-solid AD.

A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment

In this review, progress in the potential mechanisms of biochar amendment for AD performance promotion was summarized. As adsorbents, biochar was beneficial for alleviating microbial toxicity, accelerating refractory substances degradation, and upgrading biogas quality. The buffering capacity of biochar balanced pH decreasing caused by volatile fatty acids accumulation. Moreover, biochar regulated microbial metabolism by boosting activities, mediating electron transfer between syntrophic partners, and enriching functional microbes. Recent studies also suggested biochar as potential useful additives for membrane fouling alleviation in anaerobic membrane bioreactors (AnMBR). By analyzing the reported performances based on different operation models or substrate types, debatable issues and associated research gaps of understanding the real role of biochar in AD were critically discussed. Accordingly, Future perspectives of developing biochar-amended AD technology for real-world applications were elucidated. Lastly, with biochar-amended AD as a core process, a novel integrated scheme was proposed towards high-efficient energy-resource recovery from various bio-wastes.

Zerovalent Iron Effectively Enhances Medium-Chain Fatty Acids Production from Waste Activated Sludge through Improving Sludge Biodegradability and Electron Transfer Efficiency

A novel zerovalent iron (ZVI) technique to simultaneously improve the production of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) and enhance WAS degradation during anaerobic WAS fermentation was proposed in this study. Experimental results showed that the production and selectivity of MCFAs were effectively promoted when ZVI was added at 1-20 g/L. The maximum MCFAs production of 15.4 g COD (Chemical Oxygen Demand)/L and MCFAs selectivity of 71.7% were both achieved at 20 g/L ZVI, being 5.3 and 4.8 times that without ZVI (2.9 g COD/L and 14.9%). Additionally, ZVI also promoted WAS degradation, which increased from 0.61 to 0.96 g COD/g VS when ZVI increased from 0 to 20 g/L. The microbial community analysis revealed that the ZVI increased the populations of key anaerobes related to hydrolysis, acidification, and chain elongation. Correspondingly, the solubilization, hydrolysis, and acidification processes of WAS were revealed to be improved by ZVI, thereby providing more substrates (short-chain fatty acids (SCFAs)) for producing MCFAs. The mechanism studies showed that ZVI declined the oxidation-reduction potential (ORP), creating a more favorable environment for the anaerobic biological processes. More importantly, ZVI with strong conductivity could act as an electron shuttle, contributing to increasing electron transfer efficiency from electron donor to acceptor. This strategy provides a new paradigm of transforming waste sludge into assets by a low-cost waste to bring significant economic benefits to sludge disposal and wastewater treatment.

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

Two laboratory-scale up-flow anaerobic sludge blankets (UASB) reactors, one with and one without granular activated carbon (GAC), were operated for municipal sewage treatment at low temperatures (16.5 ± 2.0 °C). During the 120-day operation, the GAC-amended reactor significantly enhanced COD removal (from 62% to 75%, P < 0.05) and methane production (from 87 to 218 mg CH₄-COD/reactor/d) than the non-GAC reactor. Bacterial communities were significantly different between the two reactors (P < 0.05). Geobacter, a key indicator for direct interspecies electron transfer (DIET), had the highest differential score (LEfSe analysis), showing significantly higher abundances in the GAC-amended reactor (3.7-8.8%) than the non-GAC reactor (0.9-4.0%). GAC also enriched syntrophic bacteria, Syntrophomonas, Syntrophus and sulfate reducing bacteria. Methanobacterium dominated the archaeal community in the GAC-amended reactor sludge (35.7%) and GAC-biofilm (75.3%), and was less abundant in the non-GAC reactor (9.9%). It indicates that GAC enriched microbial syntrophic partners with potential electro-activities in the anaerobic digestion process.

Elucidating the effect of microbial inoculum and ferric chloride as additives on the removal of antibiotic resistance genes from chicken manure during aerobic composting

This experiment investigated the effect of adding a microbial inoculum (M) and ferric chloride (F) on the fate of antibiotic resistance genes (ARGs) during chicken manure composting. Adding M and F improved the microbial activity in the compost and facilitated the removal of ARGs, whereas the combined treatment achieved the best results, especially in reducing the enrichment of sul resistance genes. Tn916/1545 and intI1 were important genetic elements that affected the transfer of ARGs, and Tn916/1545 was closely related to the transfer of tetM, tetW, and ermQ in Firmicutes. Kyoto Encyclopedia of Genes and Genomes functional predictions indicated that M and F could reduce the abundance of membrane transport and signal transduction molecules in the compost products. Thus, these findings suggest that the combined application of M and F is a promising strategy that could potentially inhibit the transfer of ARGs during composting.

Dynamics of Clostridium genus and hard-cheese spoiling Clostridium species in anaerobic digesters treating agricultural biomass

Biogas plants are a widespread renewable energy technology. However, the use of digestate for agronomic purposes has often been a matter of concern. It is controversial whether biogas plants might harbor some pathogenic clostridial species, which represent a biological risk. Moreover, the inhabitance of Clostridium hard-cheese spoiling species in anaerobic digesters can be problematic for hard-cheese manufacturing industries, due to the issue of cheese blowing defects. This study investigated the effect of mesophilic anaerobic digestion processes on the Clostridium consortia distribution over time. Specifically, three lab-scale CSTRs treating agricultural biomass were characterized by considering both the whole microbial community and the cultivable clostridial spores. It is assessed an overall reduction of the Clostridium genus during the anaerobic digestion process. Moreover, it was evidenced a slight, but steady decrease of the cultivable clostridial spores, mainly represented by two pathogenic species, C. perfringens and C. bifermentans, and one hard-cheese spoiling species, C. butyricum. Thus, it is revealed an overall reduction of the clostridial population abundance after the mesophilic anaerobic digestion treatment of agricultural biomass.

Redox-based electron exchange capacity of biowaste-derived biochar accelerates syntrophic phenol oxidation for methanogenesis via direct interspecies electron transfer

In this study, six different types of biochar (based on two feedstocks and three pyrolytic temperatures) were prepared as individual additives for both syntrophic phenol degradation and methanogenesis promotion. The results showed that for phenol degradation, the addition of biochar (15 g/L) shortened the methanogenic lag time from 15.0 days to 1.1-3.2 days and accelerated the maximum CH₄ production rate from 4.0 mL/d to 10.4-13.9 mL/d. Microbial community analysis revealed that the electro-active Geobacter was enriched (from 3.8-7.7% to 11.1-23.1%), depending on the type of biochar that was added. This indicates a potential shift of syntrophic phenol metabolism from a thermodynamically unfavorable pathway with H₂ as the interspecies electron transfer mediator to direct interspecies electron transfer (DIET). Integrated analysis of methanogenesis dynamics and the electrochemical properties of biochar showed that compared with electrical conductivity, the electron exchange capacity of biochar was more likely to dominate the DIET process, which was due to the presence of redox-active organic functional groups in biochar. The removal of biochar from the anaerobic system generally prolonged the lag time, revealing the importance of adsorption capacity of biochar to mitigate bio-toxicity of phenol to microbial activity.

Iron oxide alleviates acids stress by facilitating syntrophic metabolism between Syntrophomonas and methanogens

Anaerobic digestion (AD) is a promising technology for food waste management, but frequently restricted with long lag phase as a consequent of acidification. Two laboratory experiments were conducted to investigate the effects of iron materials on food waste AD. Experiment 1 compared the effects of iron oxide (IO) and zero valent iron (ZVI) on AD performance. The results showed that both IO and ZVI could enhance methane (CH₄) generation, but IO showed better performance regarding the reduction of lag phase. The lag phase of the reactor supplemented with IO was 17.4% and 42.7% shorter than that of the reactor supplemented with ZVI and the control, respectively. Based on these results, experiment 2 was designed to examine the role of IO in alleviation of acid stress at high substrate to inoculum (SI) ratio. The results showed that supplemented IO into reactor could ensure a successful methanogenesis when operating at high SI ratio, while IO-free reactor was failed to generate CH₄ although operating for 77 days. Supplementing IO into the reactor after 48 h of digestion could restore the CH₄ generation, though its lag phase was 2.6 times of the reactor supplemented with IO at the beginning of the digestion. Microbial community structure analysis revealed that IO could simultaneously enrich Syntrophomonas and methanogens (i.e. Methanobacterium, Methanofollis and Methanosarcina), and might promote electron transfer between those two types of microbes, which were critical for achieving an effective methanogenesis.

Impact of Nanoscale Magnetite and Zero Valent Iron on the Batch-Wise Anaerobic Co-Digestion of Food Waste and Waste-Activated Sludge

As a potential approach for enhanced energy generation from anaerobic digestion, iron-based conductive nanoparticles have been proposed to enhance the methane production yield and rate. In this study, the impact of two different types of iron nanoparticles, namely the nano-zero-valent-iron particles (NZVIs) and magnetite (Fe3O4) nanoparticles (NPs) was investigated, using batch test under mesophilic conditions (35 °C). Magnetite NPs have been applied in doses of 25, 50 and 80 mg/L, corresponding to 13.1, 26.2 and 41.9 mg magnetite NPs/gTS of substrate, respectively. The results reveal that supplementing anaerobic batches with magnetite NPs at a dose of 25 mg/L induces an insignificant effect on hydrolysis and methane production. However, incubation with 50 and 80 mg/L magnetite NPs have instigated comparable positive impact with hydrolysis percentages reaching approximately 95% compared to 63% attained in control batches, in addition to a 50% enhancement in methane production yield. A biodegradability percentage of 94% was achieved with magnetite NP doses of 50 and 80 mg/L, compared to only 62.7% obtained with control incubation. NZVIs were applied in doses of 20, 40 and 60 mg/L, corresponding to 10.8, 21.5 and 32.2 mg NZVIs/gTS of substrate, respectively. The results have shown that supplementing anaerobic batches with NZVIs revealed insignificant impact, most probably due to the agglomeration of NZVI particles and consequently the reduction in available surface area, making the applied doses insufficient for measurable effect.

Comparative facilitation of activated carbon and goethite on methanogenesis from volatile fatty acids

To provide insight into direct interspecies electron transfer (DIET) via carbon-based materials and ferric oxides, the effects of three conductive materials (i.e. activated carbon (AC), iron modified activated carbon (FEAC) and goethite (FEOOH)), on methanogenesis from volatile fatty acids (VFAs) were evaluated. Under the acid stress (~4 g/L VFAs), the maximum methane yield of 266 mL/g-chemical oxygen demand (COD) was found in the FEOOH supplemented reactor, which was 48% higher than that of AC reactor. The reasons for the enhanced activity of the electron transport chain and extracellular electron transfer ability by FEOOH include: 1) the activation on iron-containing enzymes that involved in methanogenesis and acidogenesis; 2) selective enrichment on functional microorganism. The higher electron donating capacities (EDC) value of FEOOH may be a triggering factor on the growth of Syntrophomonadaceae, which perform DIET with methanogens (Methanosaeta and Methanosarcina) for the syntrophic degradation of VFAs.

New insights of enhanced anaerobic degradation of refractory pollutants in coking wastewater: Role of zero-valent iron in metagenomic functions

Coking wastewater (CWW) has long been a serious challenge for anaerobic treatment due to its high concentrations of phenolics and nitrogen-containing heterocyclic compounds (NHCs). Herein, we proposed and validated a new strategy of using zero-valent iron (ZVI) to strengthen the anaerobic treatment of CWW. Results showed that COD removal efficiencies was increased by 9.5-13.7% with the assistance of ZVI. GC-MS analysis indicated that the removal of phenolics and NHCs was improved, and the intermediate 2(1H)-Quinolinone of quinoline degradation was further removed by ZVI addition. High-throughput sequencing showed that phenolics and NHCs degraders, such as Levilinea and Sedimentibacter were significantly enriched, and the predicted gene abundance of xenobiotic degradation and its downstream metabolic pathways was also increased by ZVI. Network and redundancy analysis indicated that the decreased oxidation-reduction potential (ORP) by ZVI was the main driver for microbial community succession. This study provided an alternative strategy for strengthening CWW anaerobic treatment.

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

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

Exclusive microbially driven autotrophic iron-dependent denitrification in a reactor inoculated with activated sludge

Autotrophic iron-dependent denitrification (AIDD) is arising as a promising process for nitrogen removal from wastewater with a low carbon to nitrogen ratio. However, there is still a debate about the existence of such a process in activated sludge systems. This work provides evidence and elucidated the feasibility of autotrophic Fe(II)-oxidizing nitrate-reducing culture for nitrogen removal by long-term reactor operation, batch experimental verification, unstructured kinetic modeling and microbial community analyses. A relatively stable nitrate removal rate was achieved coupled with the oxidation of ferrous ions in 3-month operation of reactor. The kinetic modeling suggests that the iron oxidation was a growth-associated process in AIDD. Utilization of extracellular polymeric substances (and/or soluble microbial products) as electron donor for denitrification by heterotrophic denitrifiers was not mainly responsible for nitrogen removal in the reactor. After long-term operation of the reactor with activated sludge as inoculum, the enrichment culture KS-like consortium, dominated by Fe(II) oxidizer, Gallionellaceae, was successfully acclimated for autotrophic Fe(II)-oxidizing nitrate reduction. This work extents our understanding about the existence of such an autotrophic Fe(II)-oxidizing nitrate-reducing culture in both natural and engineered systems, and opens a door for its potential application in wastewater treatment.

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.

Conductive materials in anaerobic digestion: From mechanism to application

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

Micro-level evaluation of organic compounds transformation in anaerobic digestion under feast and famine conditions assisted by iron-based materials - Revealing the true mechanism of AD enhancement

Conductive materials have been applied to assist syntrophic metabolism in anaerobic digestion. However, their role in the transformation of organic compounds, particularly recalcitrant compounds, has not been revealed. In this study, iron-based materials - magnetite nanoparticles and Fe²⁺- were employed to explore their effects on the transformation of different organic matters in anaerobic system. Prompted methane production rates and quantity in iron-based materials groups were found due to the improved solubilization of organic particles, enhanced degradation of recalcitrant compounds, and maintained microbial activity under substrate-limited conditions. Specifically, the proportion of the reducing functional groups (C-C/H or CC) and O/C ratio were always significantly lower in iron-based materials supplemented groups (Fe groups) compared to Control group, despite hydrolysis was greatly enhanced in Fe groups. The greater dehydrogenation oxidation was confirmed in the presence of iron-based materials. The remaining humic-like substances (HS), a typical type of recalcitrant compound, was about 2.5 times higher in Control group (221.2 ± 5.3 mg/L-C) compared to Fe groups after 30 days degradation. By tracking the aromaticity of HS and individual compounds at molecular level, this study reveals that iron-based materials were more effective in stimulating the degradation of aliphatic moieties than the aromatic moieties of recalcitrant compounds. When readily biodegradable substrates were limited, Fe groups continued methane generation by using recalcitrant compounds (e.g. thiethylperazine and fluvoxamino acid) as carbon source, and the microbial activity was maintained according to higher relative abundance of protonated nitrogen and continuous methanogenesis activity at starvation phase.

Redox-active biochar facilitates potential electron tranfer between syntrophic partners to enhance anaerobic digestion under high organic loading rate

Sawdust-based biochar prepared (SDBC) at three pyrolytic temperatures were compared as additives to mesophilic anaerobic digestion (AD). SDBC prepared at 500 °C performed better in enhancing CH₄ production than other SDBCs. Analyzing the crucial electro-chemical characteristics of the SDBCs revealed that the excellent electron transfer capacity of SDBC was significant to stimulate methanogenesis promotion. A long-term semi-continuous operation further confirmed that adding SDBC to AD system increased the maximum organic loading rate (OLR) from 6.8 g VS/L/d to 16.2 g VS/L/d, which attributed to the extremely low volatile fatty acids (VFA) accumulation. Microbial community succession analysis found that SDBC addition altered both bacterial and archaea structure greatly. More importantly, the syntrophic and electro-active partners of Petrimonas and Methanosarcina synergistically enriched under high OLR condition were responsible for the high-efficient VFA degradation, which suggested that SDBC likely acted as redox-active mediator to facilitate direct interspecies electron transfer between the syntrophic partners for high-efficient syntrophic methanogenesis process.

Enhancement of methane production and antibiotic resistance genes reduction by ferrous chloride during anaerobic digestion of swine manure

In this study, effects of ferrous chloride (FeCl₂) addition on methane production and antibiotic resistance genes (ARGs) reduction were investigated during anaerobic digestion (AD) of swine manure. FeCl₂ could both improve the accumulative methane production and reduce the abundance of total ARGs, i.e., the maximum increase of CH₄ production of 21.5% at FC5, and the maximum ARGs reduction of 33.3% at FC25. The reduction of pathogenic bacteria and metal resistance genes (MRGs) was enhanced. Acetate and propionate utilization were intensified by enhancing H₂ utilization and direct interspecies electron transfer (DIET), where DIET was further enhanced by the reaction of the FeCl₂ and acetic acid. The bacterial community played important role in the evolution of ARGs (68.26%), which were also affected by MRGs, mobile genetic elements (MGEs), and environmental factors. Therefore, FeCl₂-based AD is a feasible and attractive way to improve methane production and ARG reduction.

Syntrophus conductive pili demonstrate that common hydrogen-donating syntrophs can have a direct electron transfer option

Syntrophic interspecies electron exchange is essential for the stable functioning of diverse anaerobic microbial communities. Hydrogen/formate interspecies electron transfer (HFIT), in which H₂ and/or formate function as diffusible electron carriers, has been considered to be the primary mechanism for electron transfer because most common syntrophs were thought to lack biochemical components, such as electrically conductive pili (e-pili), necessary for direct interspecies electron transfer (DIET). Here we report that Syntrophus aciditrophicus, one of the most intensively studied microbial models for HFIT, produces e-pili and can grow via DIET. Heterologous expression of the putative S. aciditrophicus type IV pilin gene in Geobacter sulfurreducens yielded conductive pili of the same diameter (4 nm) and conductance of the native S. aciditrophicus pili and enabled long-range electron transport in G. sulfurreducens. S. aciditrophicus lacked abundant c-type cytochromes often associated with DIET. Pilin genes likely to yield e-pili were found in other genera of hydrogen/formate-producing syntrophs. The finding that DIET is a likely option for diverse syntrophs that are abundant in many anaerobic environments necessitates a reexamination of the paradigm that HFIT is the predominant mechanism for syntrophic electron exchange within anaerobic microbial communities of biogeochemical and practical significance.

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.

Effects of nanoscale zero-valent iron on the performance and the fate of antibiotic resistance genes during thermophilic and mesophilic anaerobic digestion of food waste

The effects of nanoscale zero-valent iron (nZVI) on the performance of food waste anaerobic digestion and the fate of antibiotic resistance genes (ARGs) were investigated in thermophilic (TR) and mesophilic (MR) reactors. Results showed that nZVI enhanced biogas production and facilitated ARGs reduction. The maximum CH₄ production was 212.00 ± 4.77 ml/gVS with 5 g/L of nZVI in MR. The highest ARGs removal ratio was 86.64 ± 0.72% obtained in TR at nZVI of 2 g/L. nZVI corrosion products and their contribution on AD performance were analyzed. The abundance of tetracycline genes reduced significantly in nZVI amended digesters. Firmicutes, Chloroflexi, Proteobacteria and Spirochaetes showed significant positive correlations with various ARGs (p < 0.05) in MR and TR. Redundancy analysis indicated that microbial community was the main factor that influenced the fate of ARGs. nZVI changed microbial communities, with decreasing the abundance bacteria belonging to Firmicutes and resulting in the reduction of ARGs.

Influence of nanoscale zero-valent iron and magnetite nanoparticles on anaerobic digestion performance and macrolide, aminoglycoside, β-lactam resistance genes reduction

The effect of nanoscale zero-valent iron (NZVI) and magnetite nanoparticles (Fe₃O₄ NPs) on anaerobic digestion (AD) performance was investigated through a series of 100-day semi-continuous mesophilic anaerobic digestions. The results indicated that biogas production had increased by 24.44% and 21.66% with the addition of 0.5 g/L Fe₃O₄ NPs and 1.0 g/L NZVI, respectively. Besides, the abundance of five widespread antibiotic resistance genes (ARGs) (ermF, ermA, ermT, aac(6')-IB, blaOXA-1) was also studied. The decrease in abundance of aac(6')-IB and blaOXA-1 was observed during the AD process with an average removal rate of 95.69% and 44.82%, respectively. Most of the ARGs, especially ermA and ermT, were less abundant in NZVI group compared with control group. The overall results suggested that the addition of NZVI and Fe₃O₄ NPs contributed to a better sludge anaerobic digestion performance, and NZVI was beneficial to the removal of some ARGs.

Impact of adding metal nanoparticles on anaerobic digestion performance - A review

Anaerobic digestion is the most widely adopted biological waste treatment processes with renewable energy production. The effects of adding metal nanoparticles (NPs) on improving digestion performance are well noted. This paper reviewed the traditional view on the cytotoxicity of NPs to living organisms and the contemporary view of mechanisms for enhancement in anaerobic digestion performance in the presence of metal NPs. The complicated interactions acquire further studies for comprehending the physical and chemical interactions of metal NPs to the constituent compounds and to the living cells, and the involvement of mechanisms such as direct interspecies electron transfer for better design and control of the "NP strategy" for anaerobic digestion performance enhancement.

NanoFe3O4 accelerates methanogenic straw degradation by improving energy metabolism

The impacts of nanoFe₃O₄ on the composition of degradation products, microbial community, and microbial metabolic functions during rice straw anaerobic degradation were investigated. Under nanoFe₃O₄ addition, CH₄ production and straw degradation increased by 81% and 10.4%, respectively, in paddy soil enrichment. Coupling product chemistry and microbial community during straw degradation found that nanoFe₃O₄ effectively promoted the hydrolysis-acidification-methanogenesis of straw, which made lignin-, lipid-, protein-, tannin-like and VFAs products rapidly increase and then quickly decrease. Moreover, the relative abundance of Clostridiaceae and Methanosarcina corresponded with increased hydrolysis and acetoclastic methanogenesis with nanoFe₃O₄ addition. Cellular processes, environmental information processing and metabolism, especially energy metabolism, were enhanced functions of the microbial community during straw degradation with nanoFe₃O₄. The nanoFe₃O₄ addition may improve the electron transfer efficiency, stimulate energy release, reduce Gibbs free energy of the half reaction of organic carbon oxidation (ΔG_cox⁰) and promote energy metabolism to accelerate straw degradation and CH₄ generation.

Synergistic effect of magnetite and zero-valent iron on anaerobic degradation and methanogenesis of phenol

Anaerobic digestion is widely employed for treating phenol-containing wastewater, but there are still some drawbacks such as slow phenol degradation rate and vulnerable acetoclastic methanogens. Coupling of magnetite (Fe₃O₄) and zero valent iron (ZVI) was firstly used to enhance anaerobic digestion of phenol. The results indicated an obvious synergistic effect was generated with coupling of Fe₃O₄ and ZVI during the whole anaerobic digestion of phenol. The phenol degradation rate and methane production of Fe₃O₄/ZVI-added group were increased by 8.8-23.1% and 11.9-31.6%, respectively compared with Fe₃O₄-added group, and enhanced by 5.9-17.1% and 4.4-18.3%, respectively compared with ZVI-added group. ZVI improved the growth of hydrogenotrophic methanogens and Fe₃O₄ enhanced the growth of syntrophic acetate-oxidizing bacteria. Finally, the syntrophic interaction between acetate-oxidizing bacterium and hydrogenotrophic methanogens played a vital role on the synergistic effect of Fe₃O₄ and ZVI on the whole anaerobic phenol digestion.

Identification of parameters needed for optimal anaerobic co-digestion of chicken manure and corn stover

While studies have shown that anaerobic co-digestion of chicken manure (CM) and corn stover (CS) is an efficient method to treat these agricultural wastes, the microbial ecology of these systems and optimal parameters for the digestion process are yet to be determined. In this study, the effects of different initial substrate concentrations and CS : CM mixture ratios on co-digestion and microbial community structure were evaluated. Results demonstrated that both the highest cumulative methane yields and methane production rates were obtained from reactors with a CS : CM ratio of 1 : 1 during hemi-solid-state anaerobic digestion (HSS-AD). Cumulative methane yields and methane production rates were 24.8% and 42% lower in solid-state anaerobic digestion (SS-AD) reactors using the same CS : CM ratios. Analysis of microbial community structures revealed that cellulolytic bacteria and a diversity of syntrophic microorganisms capable of direct interspecies electron transfer (DIET) and hydrogen interspecies transfer (HIT) were enriched in the best-performing reactors. Methanosarcina species also dominated during HSS-AD, and their presence was positively correlated with methane production in the reactors.

Effects of ferric oxide on the microbial community and functioning during anaerobic digestion of swine manure

Iron-based materials have been suggested as environmentally-friendly additives that can enhance methane production during anaerobic digestion (AD). In this study, the effects of ferric oxide (Fe₂O₃) addition on methane production were investigated during swine manure AD. In addition, the effects of Fe₂O₃ addition on the AD ternary pH buffer system and microbial community were evaluated. Fe₂O₃ could improve the accumulative methane production by maximum 11.06% when adding 75 mmol of Fe₂O₃. Higher methane production could be attributed to the enhancement of direct interspecies electron transfer (DIET) and the formation of Fe-S precipitates, but not the addition of Fe₂O₃ as a nutrient. Furthermore, Fe₂O₃ addition enhanced methanogenesis rather than acetogenesis, as evinced by analysis of functional genes. Nevertheless, high-throughput sequence analysis of microbial community composition revealed the lack of a significant influence by Fe₂O₃ addition, and Fe₂O₃ addition did not significantly affect the ternary pH buffer system.

Elucidating Syntrophic Butyrate-Degrading Populations in Anaerobic Digesters Using Stable-Isotope-Informed Genome-Resolved Metagenomics

Linking the genomic content of uncultivated microbes to their metabolic functions remains a critical challenge in microbial ecology. Resolving this challenge has implications for improving our management of key microbial interactions in biotechnologies such as anaerobic digestion, which relies on slow-growing syntrophic and methanogenic communities to produce renewable methane from organic waste. In this study, we combined DNA stable-isotope probing (SIP) with genome-centric metagenomics to recover the genomes of populations enriched in 13C after growing on [13C]butyrate. Differential abundance analysis of recovered genomic bins across the SIP metagenomes identified two metagenome-assembled genomes (MAGs) that were significantly enriched in heavy [13C]DNA. Phylogenomic analysis assigned one MAG to the genus Syntrophomonas and the other MAG to the genus Methanothrix. Metabolic reconstruction of the annotated genomes showed that the Syntrophomonas genome encoded all the enzymes for beta-oxidizing butyrate, as well as several mechanisms for interspecies electron transfer via electron transfer flavoproteins, hydrogenases, and formate dehydrogenases. The Syntrophomonas genome shared low average nucleotide identity (<95%) with any cultured representative species, indicating that it is a novel species that plays a significant role in syntrophic butyrate degradation within anaerobic digesters. The Methanothrix genome contained the complete pathway for acetoclastic methanogenesis, indicating that it was enriched in 13C from syntrophic acetate transfer. This study demonstrates the potential of stable-isotope-informed genome-resolved metagenomics to identify in situ interspecies metabolic cooperation within syntrophic consortia important to anaerobic waste treatment as well as global carbon cycling.IMPORTANCE Predicting the metabolic potential and ecophysiology of mixed microbial communities remains a major challenge, especially for slow-growing anaerobes that are difficult to isolate. Unraveling the in situ metabolic activities of uncultured species may enable a more descriptive framework to model substrate transformations by microbiomes, which has broad implications for advancing the fields of biotechnology, global biogeochemistry, and human health. Here, we investigated the in situ function of mixed microbiomes by combining stable-isotope probing with metagenomics to identify the genomes of active syntrophic populations converting butyrate, a C4 fatty acid, into methane within anaerobic digesters. This approach thus moves beyond the mere presence of metabolic genes to resolve "who is doing what" by obtaining confirmatory assimilation of the labeled substrate into the DNA signature. Our findings provide a framework to further link the genomic identities of uncultured microbes with their ecological function within microbiomes driving many important biotechnological and global processes.

Impact of zero valent iron on blackwater anaerobic digestion

The source diverted blackwater treatment is receiving growing attention as an alternative to conventional energy intensive wastewater management and treatment systems. Blackwater, containing concentrated organic materials, can be anaerobically digested to recovery bioenergy. However, the methane recovery from blackwater is often inhibited by the presence of high free ammonia (FA) in blackwater. In order to improve the methane production in blackwater, nano-scale zero valent iron (nZVI, 35 nm or 50 nm) or micro-scale zero valent iron (mZVI, 200 μm) at different dosages (i.e., 0.5, 1, and 10 g/L) were applied respectively in the anaerobic digestion (AD) reactor for blackwater treatment. The results demonstrated that low doses (0.5-1 g/L) of nZVI slightly improved methane (CH₄) production, possibly due to a reduced oxidation-reduction potential (ORP) and improved hydrolysis-acidification in the nZVI supplemented systems. However, a lower biochemical methane potential (BMP) of blackwater was observed with high doses (10 g/L) of nZVI which induced a pH increase (>8.5) in AD reactor leading to a higher FA inhibition of CH₄ production. In contrast, the effect of mZVI on blackwater AD system was not significant. The study demonstrated the successful application of nZVI for improving AD of blackwater, however, which requires dosage control.

Stimulation of Smithella-dominating propionate oxidation in a sediment enrichment by magnetite and carbon nanotubes

Recent studies have shown that application of conductive materials including magnetite and carbon nanotubes (CNTs) can promote the methanogenic decomposition of short-chain fatty acids and even more complex organic matter in anaerobic digesters and natural habitats. The linkage to microbial identity and the mechanisms, however, remain poorly understood. Here, we evaluate the effects of nanoscale magnetite (nanoFe₃ O₄ ) and multiwalled CNTs on the syntrophic oxidation of propionate in an enrichment obtained from lake sediment. The microbial populations were composed mainly of Smithella, Syntrophomonas, Methanosaeta, Methanosarcina and Methanoregula. In addition to acetate, butyrate was transiently accumulated indicating that propionate was oxidized by Smithella via the dismutation pathway and part of the leaked butyrate was oxidized by Syntrophomonas. Propionate oxidation and CH₄ production were significantly accelerated in the presence of nanoFe₃ O₄ and CNTs. While propionate oxidation was suppressed upon H₂ application and suspended completely upon formate application in the control, this suppressive effect was substantially compromised in the presence of nanoFe₃ O₄ and CNTs. The tests on hydrogenotrophic methanogenesis of a pure culture methanogen and of the enrichment culture without propionate showed negative effect by both materials. The positive effect of nanoFe₃ O₄ disappeared when it was insulated by surface-coating with silica. Observations made with fluorescence in situ hybridization and scanning electron microscope indicated the extensive formation of microbial cell-conductive material mixture aggregates. Our results suggest that direct interspecies electron transfer is likely activated by the conductive materials and operates in concert with H₂ /formate-dependent electron transfer for syntrophic propionate oxidation in the sediment enrichment.

Response and mechanisms of the performance and fate of antibiotic resistance genes to nano-magnetite during anaerobic digestion of swine manure

Swine manure is an important reservoir of environmental antibiotic resistance genes (ARGs), and anaerobic digestion (AD) is a commonly used method for swine manure treatment. In this study, the optimized dosage of nano-magnetite to enhance methane production was figured out, the changes of the fate of ARGs response to nano-magnetite were investigated, and the microbial mechanisms were deciphered through the microbial community analysis and key functional genes quantification. Results showed that nano-magnetite could improve the methane production by maximum 6.0%, the maximum daily methane production could be increased by 47.8%, and the AD time could be shortened by above 20.0% at the addition of 75 mmol. The improved performance could be associated with the enhancement of direct interspecies electron transfer (DIET) and the inhibition release due to the formation of Fe-S precipitation not the nutrition elements role of nano-magnetite, and nano-magnetite did not significantly influence the dynamics of microbial community. Nano-magnetite could enhance the methanogenesis instead of the acetogenesis reflected by the functional genes analysis, and the limited effects of nano-magnetite on the fate of ARGs could be associated with its limited influence on the microbial community which determined the fate of ARGs during AD of swine manure.

Nanoferrosonication: A novel strategy for intensifying the methanogenic process in sewage sludge

In this work, the effect of coupling ultrasonic pretreatment with dosing of zero-valent iron nanoparticles (nanoferrosonication, "NFS") to improve the anaerobic digestion of sewage sludge was studied. Biochemical methane potential tests were conducted at 15,000 and 25,000 kJ/kg_TS and their combinations with 2 and 7 mgFe⁰/g_VS. The biogas yield increased from 106 (control) to 143 (25,000 kJ/kg_TS) and 308 mL/g_VS with NFS (7 mgFe⁰/g_VS + 15,000 kJ/kg_TS). The methane content increased from 55.6 to 66%, and the maximum VS removal was 11.5% at 7 mgFe⁰/g_VS + 15,000 kJ/kg_TS. The results demonstrated that NFS was effective in intensifying the process.

Enhancing the performance and stability of the anaerobic digestion of sewage sludge by zero valent iron nanoparticles dosage

This work studied the effects on the anaerobic digestion of sewage sludge by zero valent iron nanoparticles (NZVI) dosage. Biochemical methane potential tests were carried out with 5-9 mg/g_VS (99.7%, 40-60 nm). The biogas yield increased from 132 (control) to 310 mL/g_VS with 9 mg/g_VS. The methane content increased from 63.2% (control) to 77.6% with NZVI, which corresponded to a maximum yield of 238 mLCH₄/g_VS with 9 mg/g_VS. The maximum VS reduction was 19.6%. The highest INT-ETS activity (20.1-37.1 µgINT_red/g_VS·h) corresponding to the maximum values of sCOD was reached within the first days. NZVI decreased the ORP to -300 mV and increased the VFA's concentration (+2000 mg/L). The ORP-VFA-pH analysis showed that NZVI promoted the acidogenesis-acetogenesis without acidification. That is, NZVI was effective in intensifying the performance and stability of the process.

Effects of nanoscale zero valent iron (nZVI) concentration on the biochemical conversion of gaseous carbon dioxide (CO2) into methane (CH4)

This study presents the effects of nanoscale zero valent iron (nZVI) concentration on the biomethanation of gaseous CO₂. During anaerobic batch experiment with 9 times injection of CO₂, the CO₂ concentration in the headspace rapidly decreased by dissolution. Then, when nZVI was added at 6.25 and 12.5 g/L, the dissolved CO₂ was biochemically transformed into CH₄ at a maximum production rate of 2.38 and 3.93 μmol/hr, respectively. Biomethanation at these two nZVI concentrations continued until the end of experiment. In spite of more H₂ evolution by nZVI at 25 g/L, biomethanation did not occur, due to the significant inhibition of methanogenesis by nZVI. As the nZVI concentration increased, relative abundance of the hydrogenotrophic methanogens, especially Methanobacteriales, increased. However, at 25 g/L of nZVI concentration, acetic acid was accumulated and the relative abundance of Clostridium became predominant, indicating that homoacetogenesis was superior over methanogenesis.

Effects of individual and combined zinc oxide nanoparticle, norfloxacin, and sulfamethazine contamination on sludge anaerobic digestion

This work investigated the individual and combined effects of zinc oxide, norfloxacin, and sulfamethazine on sludge anaerobic digestion-associated methane production, protein and carbohydrate metabolism, and microbial diversity. Norfloxacin and sulfamethazine (500 mg/kg) did not inhibit methane production, but inhibited its production rate. Zinc oxide nanoparticles with antibiotics inhibited hydrolysis, fermentation, and methanogenesis over varying digestion periods. Complex pollution had a greater impact on methane production than zinc oxide alone, with acute, synergistic toxicity to methanogenesis over short periods. Complex pollution also had varying effects on bacterial and archaeal communities during digestion. These results aid understanding of the toxicity of emerging contaminants in sludge digestion, with the potential to improve pollution removal and reduce associated risks.

Graphite-assisted electro-fermentation methanogenesis: Spectroelectrochemical and microbial community analyses of cathode biofilms

The stimulatory effect of conductive particles on anaerobic digestion has been demonstrated in recent years. However, it is yet to be determined whether and how conductive particles affect methanogenesis via electro-fermentation (electro-fermentation methanogenesis). In this study, it was demonstrated, for the first time, that conductive graphite boosted the methane production yield by 54.3% and increased the maximum methane production rate by 72.2% during electro-fermentation methanogenesis. Graphite significantly affected the composition of cathode biofilms, with more live and large aggregates being observed. Spectroelectrochemical analyses further showed that the kinds and intensities of biocatalytic active sites and redox groups on the cathode biofilms increased during graphite-assisted electro-fermentation methanogenesis. Particularly, c-type cytochromes, humic acid-like substances, and humic substances improved the long-range electron transport to methanogens such as Methanobacterium and Methanosarcina. The results have implications for the improvement of electro-fermentation process and the use of conductive materials for biofuel recovery.

What kind of effects do Fe2O3 and Al2O3 nanoparticles have on anaerobic digestion, inhibition or enhancement?

Fe₂O₃ and Al₂O₃ nanoparticles are widely used in products and find their way to wastewater treatment plants through the contact of water with these products. In this study, impacts of Fe₂O₃ and Al₂O₃ nanoparticles on methane potential of waste activated sludge (WAS) were investigated by comparing long and short term toxicity test results, modelling and FISH analysis. Methane production from the samples treated with the maximum concentration of Fe₂O₃ nanoparticles decreased 28.9% at the end of the long term BMP test. EC₅₀ value for BMP test of the Fe₂O₃ nanoparticles was calculated as 901.94 mg/gTS with high coefficient of determination. Methane production from the samples treated with Al₂O₃ nanoparticles increased up to 14.8% (p > 0.05) at the end of the BMP test. However, short term toxicity tests for Fe₂O₃ and Al₂O₃ nanoparticles showed no impact on anaerobic digestion of WAS. Kinetic parameters obtained from models and captured FISH images were consistent with these results. Different impacts of nanoparticles on methane production suggested that anaerobic microorganisms can be affected from nanoparticles in various mechanisms. Hydrolysis (k_H) and overall reaction rates (k_R) values were determined as 0.0277 and 0.1441 d^-1, respectively for each concentration of Al₂O₃ nanoparticles and raw WAS. Similarly, methane production from WAS containing 5, 50, 150 and 250 mgFe₂O₃/gTS were modeled with same kinetic values. However, k_H constant was calculated as 0.0149 d^-1 for 500 mgFe₂O₃/gTS. This means that Fe₂O₃ nanoparticles starting from this concentration inhibited the methanogenic consortium and caused decreased biogas production and spesific methane production rate.

Improving the co-digestion performance of waste activated sludge and wheat straw through ratio optimization and ferroferric oxide supplementation

Low anaerobic digestion efficiency of wheat straw (WS) has been an intractable problem owing to its high C/N ratio and complex structure. In this study, co-digestion of WS and waste activated sludge (WAS) at different ratios was performed to identify conditions that would elevate the acidic pH and increase methane production. The results showed that using a 1:1 ratio of WS and WAS, methane production in the co-digester was 26.9% higher than the sum of equal WAS and WS mono-digestion. When Fe₃O₄ was added to the co-digester, the acidic pH was further relieved and the anaerobic digestion efficiency was additionally enhanced. Microbial analysis showed that the ethanol-type fermentative bacterial genus Ethanoligenens was enriched in the WAS + WS-Fe₃O₄ reactor, in which the production of propionate was notably reduced, indicating that Fe₃O₄ could prevent the accumulation of volatile fatty acids by changing the types of fermentative bacteria present and promote anaerobic digestion efficiency.