Long-term impact of nano zero-valent iron on methanogenic activity, microbial community structure, and transcription activity in anaerobic wastewater treatment system

Nano zero-valent iron (NZVI) is commonly used in industrial wastewater treatment. However, its long-term impact mechanisms of metabolization in anaerobic systems are not well understood. This study investigated the effects of long-term and continuous addition of NZVI on methanogenic activity, microbial community, and transcription activity. The results demonstrated that low levels of NZVI (1000 mg/L) induced inhibition of methanogenesis after 80 days, while high levels of NZVI (5000 mg/L) immediately led to a sharp decrease of cumulative methane production and chemical oxygen demand removal, which arrived at a steady state (14.4 % of control and 17 %) after 30 days. NZVI adversely affected cell viability, adenosine triphosphate production, and fatty acid evolution of cell membranes played a crucial role in resisting chronic NZVI toxicity. Moreover, high NZVI levels hindered the transcription of key enzymes CoM and mcrA, while low NZVI levels maintained its high CoM and mcrA activity, but down-regulated the transcription of cdh and hdr. Besides, amino-utilizing bacteria was reduced under the high NZVI concentration, while low NZVI changed dominant genus with potential protein hydrolysis function from Candidatus Cloacamonas to Sedimentibacter. These results provide a guideline for proper NZVI utilization in wastewater treatment.

Utilization of nanoparticles for biogas production focusing on process stability and effluent quality

Abstract

One of the most important techniques for converting complex organic waste into renewable energy in the form of biogas and effluent is anaerobic digestion. Several issues have been raised related to the effectiveness of the anaerobic digestion process in recent years. Hence nanoparticles (NPs) have been used widely in anaerobic digestion process for converting organic wastes into useful biogas and effluent in an effective way. This review addresses the knowledge gaps and summarizes recent researchers’ findings concentrating on the stability and effluent quality of the cattle manure anaerobic digestion process using single and combinations nanoparticle. In summary, the utilization of NPs have beneficial effects on CH4 production, process optimization, and effluent quality. Their function, as key nutrient providers, aid in the synthesis of key enzymes and co-enzymes, and thus stimulate anaerobic microorganism activities when present at an optimum concentration (e.g., Fe NPs 100 mg/L; Ni NPs 2 mg/L; Co NPs 1 mg/L). Furthermore, utilizing Fe NPs at concentrations higher than 100 mg/L is more effective at reducing H2S production than increasing CH4, whereas Ni NPs and Co NPs at concentrations greater than 2 mg/L and 1 mg/L, respectively, reduce CH4 production. Effluent with Fe and Ni NPs showed stronger fertilizer values more than Co NPs. Fe/Ni/Co NP combinations are more efficient in enhancing CH4 production than single NPs. Therefore, it is possible to utilize NPs combinations as additives to improve the effectiveness of anaerobic digestion.

Article highlights

The Widespread Use of Nanomaterials: The Effects on the Function and Diversity of Environmental Microbial Communities

In recent years, as an emerging material, nanomaterials have rapidly expanded from laboratories to large-scale industrial productions. Along with people's productive activities, these nanomaterials can enter the natural environment of soil, water and atmosphere through various ways. At present, a large number of reports have proved that nanomaterials have certain toxic effects on bacteria, algae, plants, invertebrates, mammalian cell lines and mammals in these environments, but people still know little about the ecotoxicology of nanomaterials. Most relevant studies focus on the responses of model strains to nanomaterials in pure culture conditions, but these results do not fully represent the response of microbial communities to nanomaterials in natural environments. Over the years, the effect of nanomaterials infiltrated into the natural environment on the microbial communities has become a popular topic in the field of nano-ecological environment research. It was found that under different environmental conditions, nanomaterials have various effects on the microbial communities. The medium; the coexisting pollutants in the environment and the structure, particle size and surface modification of nanomaterials may cause changes in the structure and function of microbial communities. This paper systematically summarizes the impacts of different nanomaterials on microbial communities in various environments, which can provide a reference for us to evaluate the impacts of nanomaterials released into the environment on the microecology and has certain guiding significance for strengthening the emission control of nanomaterials pollutants.

Coupled mechanism of enhanced and inhibitory effects of nanoscale zero-valent iron on methane production and antibiotic resistance genes in anaerobic digestion of swine manure

In this study, the turning point for nanoscale zero-valent iron's (NZVI) promotion and inhibition effects of methane production coupled with the reduction of antibiotic resistance genes (ARGs) was investigated. Adding 150 mmol/L NZVI increased methane production by maximum of 23.8 %, which was due to the chemical reaction producing H₂ and enhancement of direct interspecies electron transfer (DIET) by NZVI. NZVI350 dramatically repressed methane generation by 48.0 %, which might be associated with the large quantity of reactive oxygen species (ROS) and excessive H₂ inhibiting the functioning of microorganisms. The fate of ARGs was significantly related to daily methane production, indicating that the more methane production finally generated, the less the abundance of ARGs at last left. The reduction of ARGs was enhanced by maximum of 61.0 %, which was attributed to the inhibition of vertical gene transfer (VGT) and horizontal gene transfer (HGT) caused by steric hindrance associated with NZVI corrosion.

Enhanced biogas biological upgrading from kitchen wastewater by in-situ hydrogen supply through nano zero-valent iron corrosion

The in-situ hydrogen supply by nano zero-valent iron (nZVI, nFe⁰) corrosion provided a feasible way to improve the efficiency of biogas biological upgrading. This work studied the effects of nZVI at different dosages (0, 2, 4, 6, 8 and 10 g/L) on anaerobic digestion of kitchen wastewater by two buffer systems 2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid (HEPES) and sodium hydrogen carbonate (NaHCO₃). The addition of nZVI improved the content of methane (CH₄) and stability of anaerobic digestion process. In HEPES buffer system, the CH₄ was all increased and the maximum reached 90.51% with 10 g/L nZVI, higher than 32.25% compared to the control. The maximum hydrogen enrichment (HE) was 113 ppb after nZVI addition, indicating the mass transfer efficiency of hydrogen (H₂) was improved. Microbial community analysis showed that the total relative abundance of Methanobacterium and Methanolinea at 10 g/L nZVI was 53.72%, which was 1.62 times of the control group. However, in the NaHCO₃ buffer system with 10 g/L nZVI addition, the content of CH₄ and the loosely bound extracellular polymeric substances (LB-EPS) was lower than the control. The results indicated that the addition of nZVI was feasible for biogas upgrading, and the bidirectional effect of nZVI on the promotion or inhibition of bio-methanation might be related to the buffer system of the anaerobic process.

Novel application of pyrolytic carbon generated from waste tires: Hydrolytic and methanogenic performance promotion in vinegar residue anaerobic digestion

Random disposal of waste tires and vinegar residues is deleterious to the environment; these materials can be sufficiently treated using pyrolysis and anaerobic digestion, respectively. In this study, pyrolytic carbon was used to enhance the performance of the anaerobic digestion of vinegar residues, which is a much more economic method comparing with dosing commercial-level carbon based materials. The conductivity of pyrolytic carbon at 1000 °C is much higher than that of commercial activated carbon. At a dosage of 10 g per 29 g of vinegar residues, the maximum volatile fatty acid production was 4225.4 mg COD/L in the reactor (effective volume of 400 mL) with inoculum to substrate ratio (ISR) of 1:1, representing an increase of 50.3% from that of the control reactor. A sufficient dosage is necessary to improve methane yield. The maximum methane yield was obtained at a pyrolytic carbon dosage, obtained at 1000 °C, of 12 g per 29 g of vinegar residues. The results indicated that the differences in the microbial communities of the control and experimental reactors correlated with the performance; however, the deep microbial mechanism of pyrolytic carbon boosting anaerobic digestion performance must be explored in further studies.

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

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

Synthesis, Characterization, and Synergistic Effects of Modified Biochar in Combination with α-Fe2O3 NPs on Biogas Production from Red Algae Pterocladia capillacea

This study is the first work that evaluated the effectiveness of unmodified (SD) and modified biochar with ammonium hydroxide (SD-NH2) derived from sawdust waste biomass as an additive for biogas production from red algae Pterocladia capillacea either individually or in combination with hematite α-Fe2O3 NPs. Brunauer, Emmett, and Teller, Fourier transform infrared, thermal gravimetric analysis, X-ray diffraction, transmission electron microscopy, Raman, and a particle size analyzer were used to characterize the generated biochars and the synthesized α-Fe2O3. Fourier transform infrared (FTIR) measurements confirmed the formation of amino groups on the modified biochar surface. The kinetic research demonstrated that both the modified Gompertz and logistic function models fit the experimental data satisfactorily except for 150 SD-NH2 alone or in combination with α-Fe2O3 at a concentration of 10 mg/L. The data suggested that adding unmodified biochar at doses of 50 and 100 mg significantly increased biogas yield compared to untreated algae. The maximum biogas generation (219 mL/g VS) was obtained when 100 mg of unmodified biochar was mixed with 10 mg of α-Fe2O3 in the inoculum.

Perspectives on microbial community in anaerobic digestion with emphasis on environmental parameters: A systematic review

This paper review is aiming to comprehensively identify and appraise the current available knowledge on microbial composition and microbial dynamics in anaerobic digestion with focus on the interconnections between operational parameters and microbial community. We systematically searched Scopus, Web of Science, pubmed and Embase (up to August 2019) with relative keywords to identify English-language studies published in peer-reviewed journals. The data and information on anaerobic reactor configurations, operational parameters such as pretreatment methods, temperature, trace elements, ammonia, organic loading rate, and feedstock composition and their association with the microbial community and microbial dynamics were extracted from eligible articles. Of 306 potential articles, 112 studies met the present review objectives and inclusion criteria. The results indicated that both aceticlastic and hydrogenotrophic methanogenesis are dominant in anaerobic digesters and their relative composition is depending on environmental conditions. However, hydrogenotrophic methanogens are more often observed in extreme conditions due to their higher robustness compared to aceticlastic methangoens. Firmicutes and Bacteroidetes phyla are most common fermentative bacteria of the acidogenic phase. These bacteria secrete lytic enzymes to degrade organic matters and are able to survive in extreme conditions and environments due to their spores. In addition, among archaea Methanosaeta, Methanobacterium, and Methanosarcinaceae are found at high relative abundance in anaerobic digesters operated with different operational parameters. Overall, understanding the shifts in microbial composition and diversity as results of operational parameters variation in anaerobic digestion process would improve the stability and process performance.

The application status, development and future trend of nano-iron materials in anaerobic digestion system

Growing environment problem and emphasis of environmental protection motivate intense research efforts in exploring technology to improve treatment efficiency on refractory organic pollutants. Hence, finding a method to make up for the deficiency of anaerobic digestion (AD) is very attractive and challenging tasks. The recent spark in the interest for the usage of some nanomaterials as an additive to strengthen AD system. The adoption of iron compounds can influence the performance and stability in AD system. However, different iron species and compounds can influence AD system in significantly different ways, both positive and negative. Therefore, strengthening mechanism, treatment efficiency, microbial community changes in Nanoscale Zero Valent Iron (nZVI) and Fe₃O₄ nanoparticles (Fe₃O₄ NPs) added AD systems were summarized by this review. The strengthening effects of nZVI and Fe₃O₄ NPs in different pollutants treatment system were analyzed. Previous study on the effects of nZVI and Fe₃O₄ NPs addition on AD have reported the concentration of nZVI and Fe₃O₄ NPs, and the types and biodegradability of pollutants might be the key factors that determine the direction and extent of effect in AD system. This review provides a summary on the nZVI and Fe₃O₄ NPs added AD system to establish experiment systems and conduct follow-up experiments in future study.

Impact of metallic nanoparticles on anaerobic digestion: A systematic review

Anaerobic digestion (AD) is one of the most energy-efficient waste treatment technologies for biodegradable wastes. Owing to the increasing trend of metallic nanoparticle applications in industry, they are ubiquitous to the waste streams, which may lead to remarkable impacts on the performance of the AD process. This review addresses the knowledge gaps and summarises the findings from the academic articles published from 2010 to 2019 focusing on the influences on both AD processes of biochemical hydrogen-generation and methane-production from selected metallic nano-materials. Both qualitative and quantitative analyses were conducted with selected indicators to evaluate the metallic nanoparticles' influences on the AD process. The selected metallic nanoparticles were grouped in the view of their chemical formulations aiming to point out the possible mechanisms behind their effects on AD processes. In summary, most metallic nanoparticles with trace-element-base (e.g. iron, cobalt, nickel) have positive effects on both AD hydrogen-generation and methane-production processes in terms of gas production, effluent quality, as well as process optimisation. Within an optimum concentration, they serve as key nutrients providers, aid key enzymes and co-enzymes synthesis, and thus stimulate anaerobic microorganism activities. As for the nano-additives without trace-element base, their positive influences are relied on providing active sites for the microorganism, as well as absorbing inhibitory factors. Moreover, comparisons of these nano-additives' impacts on the two gas-production phases were conducted, while methane-production phases are found to be more sensitive to additions of these nanoparticles then hydrogen-production phase. Research perspectives and research gaps in this area are discussed.

Performance prediction of ZVI-based anaerobic digestion reactor using machine learning algorithms

The use of zero-valent iron (ZVI) to enhance anaerobic digestion (AD) systems is widely advocated as it improves methane production and system stability. Accurate modeling of ZVI-based AD reactor is conducive to predicting methane production potential, optimizing operational strategy, and gathering reference information for industrial design in place of time-consuming and laborious tests. In this study, three machine learning (ML) algorithms, namely random forest (RF), extreme gradient boosting (XGBoost), and deep learning (DL), were evaluated for their feasibility of predicting the performance of ZVI-based AD reactors based on the operating parameters collected in 9 published articles. XGBoost demonstrated the highest accuracy in predicting total methane production, with a root mean squared error (RMSE) of 21.09, compared to 26.03 and 27.35 of RF and DL, respectively. The accuracy represented by mean absolute percentage error also showed the same trend, with 14.26%, 15.14% and 17.82% for XGBoost, RF and DL, respectively. Through the feature importance generated by XGBoost, the parameters of total solid of feedstock (TS_f), sCOD, ZVI dosage and particle size were identified as the dominant parameters that affect the methane production, with feature importance weights of 0.339, 0.238, 0.158, and 0.116, respectively. The digestion time was further introduced into the above-established model to predict the cumulative methane production. With the expansion of training dataset, DL outperformed XGBoost and RF to show the lowest RMSEs of 11.83 and 5.82 in the control and ZVI-added reactors, respectively. This study demonstrates the potential of using ML algorithms to model ZVI-based AD reactors.

The bio-chemical cycle of iron and the function induced by ZVI addition in anaerobic digestion: A review

Zero-valent iron (ZVI) is known to be an additive in facilitating waste treatment and improving biogas production in anaerobic digestion (AD) systems. This review concentrates on the chemical cycle of iron as well as the function of the iron cycle in the removal of four kinds of pollutants: organic carbon, nitrogen, sulphur and phosphorus, which are commonly encountered in waste treatment. In recent studies, the addition of ZVI to an AD system promoted the in-situ production of CH₄ from CO₂, enabling carbon capture through biotechnology. Additionally, using iron-carbon microbial electrolytic cells in AD systems in order to accelerate electron transport, as well as specific pollutant degradation mechanisms, are illustrated in the present study. Particularly, the main factors affecting the removal efficiency of contaminants in a ZVI-AD system such as pH, VFA/ Alkalinity (ALK), oxidation-reduction potential and particle size are reviewed. According to the above characteristics, combined with technical model and economic analyses, an AD system based on ZVI was considered to be an economical, efficient and carbon-neutral pollutant treatment technology. Accordingly, Iron-based AD is suggested to be a promising and sustainable approach orientated to a circular economy, which may be applied to many waste treatments fields.

Effect of nanoscale zero-valent iron on the change of sludge anaerobic digestion process

Nanoscale zero-valent iron (NZVI), which is increasingly used for environmental remediation, has potential positive impact on Anaerobic Digestion (AD). In this study, the changes of three phases in the AD of sludge have been analysed. The addition of NZVI (1000 mg/L) in the experimental reactors resulted in an enhanced operational performance, i.e. TCOD, SCOD removal and biogas production increased by 23.13%, 12.60% and 29.55% respectively. The addition of NZVI also kept pH at an appropriate level, enhanced the use of volatile fatty acids (VFAs) and the activity of the characteristic enzyme relative to the blank group. The Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) were employed to investigate the changes in surface morphology and elements of the sludge. The infrared spectra analysis showed that NZVI promoted AD of sludge. The results from the three-phase change reflected that the addition of NZVI not only promoted AD of sludge but also improved the resource utilization of sludge.

Zero-valent iron enhanced anaerobic digestion of pre-concentrated domestic wastewater for bioenergy recovery: Characteristics and mechanisms

Pre-concentrated domestic wastewater (PDWW) rich in organic matters can be a suitable substrate for anaerobic digestion (AD) towards holistic resource and bioenergy recovery. Micron zero-valent iron (ZVI) was applied in designed batch experiments during anaerobic treatment of PDWW to verify its roles in performance enhancement and associated mechanisms. In the selected range of food to microorganism (F/M) ratio, 0.5 gCOD/gMLVSS was most appropriate as biomethane production potential (BMP) of 0.275 L CH₄/gCOD was obtained. The optimal ZVI dosage at fixed F/M of 0.5 was 6 g/L, further enhancing the BMP by 15.2%. Furthermore, ZVI improved the hydrolysis process (producing more soluble organics) and regulated acidification process (affecting volatile fatty acids distribution). No obvious impact on acetoclastic and hydrogenotrophic methanogenesis processes was noted with ZVI addition. ZVI based AD of the PDWW is promising for promoting the practical application of advanced domestic wastewater treatment strategy (pre-concentration plus anaerobic digestion).

Enhanced electricity generation in rice paddy-field microbial fuel cells supplemented with iron powders

Microbial fuel cells installed in rice paddy fields (RP-MFCs) are able to serve as on-site batteries for operating low-power environmental sensors. In order to increase the utility and reliability of RP-MFCs, however, further research is necessary for boosting the power output. Here we examined several powdered iron species, including zero valent iron (ZVI), goethite, and magnetite, for their application to increasing power outputs from RP-MFCs. Soil around anodes was supplemented with either of these iron species, and RP-MFCs were operated for several months during the transplanting and harvesting. It was found that power outputs from RP-MFCs supplemented with ZVI were more than double the outputs from control (not supplemented with iron species) and other RP-MFCs, even after iron corrosion was ceased, and the maximum power density reached 130 mW/m² (per projected area of the anode). Metabarcoding of 16S rRNA gene amplicons suggested that several taxa represented by fermentative and exoelectrogenic bacteria were substantially increased in MFCs supplemented with ZVI. Results suggest that ZVI lowers oxidation/reduction potential around anodes, activates anaerobic microbes involved in the conversion of organic matter into electricity and increases power output from RP-MFCs.

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.

Enhancement of methanogenic activity in anaerobic digestion of high solids sludge by nano zero-valent iron

This study evaluated the effects of nano zero-valent iron (NZVI, 50 nm) on anaerobic digestion of high solids sludge (10 ± 0.5%). Compared to the blank group without NZVI, the group with NZVI at all levels (10, 20 and 30 mM) played a driving role in methane production. The maximal methane production was increased by 37.5% in the group of 30 mM NZVI. The dynamic changes of hydrogen content and VFAs showed that rapid hydrogen evolutional corrosion of NZVI made lower hydrogen partial pressure in the later stage, which was more conducive to conversion of propionic acid. The microscopic analysis indicated that NZVI could flocculate and adsorb on the extracellular polymeric substances (EPS) around the anaerobic microorganisms, protecting most active microbial cell membrane from contact damage. On the other hand, some decaying microbial cells membrane could be destroyed by NZVI and intracellular substances would be released due to the reduction of EPS.

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.

Effects of nano-zerovalent iron on antibiotic resistance genes during the anaerobic digestion of cattle manure

This study investigated the effects of adding nano-zerovalent iron (nZVI) at three concentrations (0, 80, and 160 mg/L) on the methane yield and the fate of antibiotic resistance genes (ARGs) during the anaerobic digestion (AD) of cattle manure. The addition of nZVI effectively enhanced the methane yield, where it significantly increased by 6.56% with 80 mg/L nZVI and by 6.43% with 160 mg/L nZVI. The reductions in the abundances of ARGs and Tn916/1545 were accelerated by adding 160 mg/L nZVI after AD. Microbial community analysis showed that nZVI mainly increased the abundances of bacteria with roles in hydrolysis and acidogenesis, whereas it reduced the abundance of Acinetobacter. Redundancy analysis indicated that the changes in mobile genetic elements made the greatest contribution to the fate of ARGs. The results suggest that 160 mg/L nZVI is a suitable additive for reducing the risks due to ARGs in AD.

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.

A Review on Nanoparticles as Boon for Biogas Producers—Nano Fuels and Biosensing Monitoring

Nanotechnology has an increasingly large impact on a broad scope of biotechnological, pharmacological and pure technological applications. Its current use in bioenergy production from biomass is very restricted. The present study is based on the utilization of nanoparticles as an additive to feed bacteria that break down natural substances. The novel notion of dosing ions using modified nanoparticles can be used to progress up biogas production in oxygen free digestion processes. While minute nanoparticles are unstable, they can be designed to provide ions in a controlled approach, so that the maximum enhancement of biogas production that has been reported can be obtained. Nanoparticles are dissolved in a programmed way in an anaerobic atmosphere and are supplied in a sustainable manner to microbiotic organisms responsible for the degradation of organic material, which is a role that fits them well. Therefore, biogas fabrication can be increased up to 200%, thereby increasing the degradation of organic waste.

Comparison of enhancement of anaerobic digestion of waste activated sludge through adding nano-zero valent iron and zero valent iron

The feasibility of adding nano-zero valent iron (NZVI: 0.6, 1.0, 4.0, 10.0 g L⁻¹) to enhance anaerobic digestion of waste activated sludge (WAS) was examined by comparison with ZVI, and the mechanisms of NZVI enhancement of the hydrolysis and methanogenesis processes were elucidated. NZVI could enhance hydrolysis–acidification of WAS by destroying the integrity of microbial cells. Both volatile fatty acids production and the acetic acid portion were greatly improved by NZVI additions, peaking at 4.0 g L⁻¹ NZVI. In anaerobic digestion, CH₄ production was promoted at a NZVI dosage ≤1.0 g L⁻¹. The optimum dosage of NZVI for methanogenesis was 1.0 g L⁻¹, and further addition of NZVI could cause inhibition of methanogenesis because of long-term accumulation of H₂. ZVI could also improve hydrolysis–acidification and the CH₄ yield, but its efficiency was relatively low compared with NZVI, and it could not induce cell wall rupture. 16S rDNA high-throughput sequencing results showed that NZVI addition at appropriate dosage facilitated increasing the proportion of microorganisms involved in WAS hydrolysis–acidification and methanogenesis.

The feasibility of adding nano-zero valent iron to enhance anaerobic digestion of waste activated sludge was studied by comparison with ZVI, and the mechanisms of NZVI enhancement of the hydrolysis and methanogenesis processes were elucidated.