Effect of microscale ZVI/magnetite on methane production and bioavailability of heavy metals during anaerobic digestion of diluted pig manure

Closed fixed-bed bacteria-algae biofilm reactor: A promising solution for phenol containing wastewater treatment and resource transformation

This study focuses on treating phenolic wastewater with a novel closed fixed-bed bacteria-algae biofilm reactor (CF-BABR) to enhance resource transformation for phenolic substances. The CF-BABR showed strong impact - load resistance and stable degradation efficiency, fully degrading phenolic compounds at concentrations from 0 to 150 mg/L. From the inflow to the outflow, the effective sequences, abundance, and diversity of bacteria decreased. Chlorobaculum was the dominant bacterium for phenolic pollutant degradation. The abundance of fungi decreased gradually, while their diversity increased. Kalenjinia and Cutaneotrichosporon played a synergistic role in reducing pollutant toxicity. The high - concentration pollutants at the influent led to a higher abundance of microalgal communities, and Scenedesmaceae became the most dominant algal family, which was positively correlated with the degradation of phenolic compounds. Functional gene prediction indicated that the abundance of functional genes in bacteria decreased overall along the wastewater flow. Carbohydrate metabolism and amino acid metabolism were the most active secondary pathways. In fungi, the predicted gene functions had the highest abundance in the upstream region. Metabolic intermediates such as organic acids and derivatives, lipids and lipid - like molecules, and carboxylic acids and derivatives demonstrated the degradation effect of CF-BABR on phenolic compounds.

Comparing the effects of magnetite-mediated direct interspecies electron transfer with biogas mixing-driven interspecies hydrogen transfer on anaerobic digestion

Although the promotive effect of direct interspecies electron transfer (DIET) on methane production has been well-documented, the practical applicability of DIET in different scenarios have not yet been systematically studied. This study compared the effects of magnetite-mediated DIET with conventional biogas mixing-driven interspecies hydrogen transfer (IHT) on anaerobic digestion (AD) of swine manure (SM). Compared with control, magnetite supplementation, biogas circulation, and their integration enhanced the CH4 yield by 19.3%, 25.9%, and 26.2%, respectively. Magnetite mainly enriched DIET-related syntrophic bacteria (Anaerolineae and Synergistia) and methanogens (Methanosarcina) to accelerate acidification and establish DIET, while biogas circulation mainly enriched hydrolytic bacteria (Clostridia) and hydrogenotrophic methanogens (Methanolinea and Methanobacterium) to promote hydrolysis and accelerate IHT. Coupling magnetite addition with biogas circulation led to the enrichment of the above six microorganisms to different extents. The effectiveness of the strategies for lowering the H2 pressure followed: magnetite + biogas circulation ≈ biogas circulation > magnetite. Under stress-free environment, the enhancement effect of magnetite-induced DIET was not even as pronounced as biogas circulation-a simple and common mixing strategy in commercial AD plants, and the promotion effect of magnetite was insignificant in the well-mixed digesters. In short, the magnetite-mediated DIET is not always effective in improving AD of SM.

A review on sulfur transformation during anaerobic digestion of organic solid waste: Mechanisms, influencing factors and resource recovery

Anaerobic digestion (AD) is an economical and environment-friendly technology for treating organic solid wastes (OSWs). OSWs with high sulfur can lead to the accumulation of toxic and harmful hydrogen sulfide (H₂S) during AD, so a considerable amount of studies have focused on removing H₂S emissions. However, current studies have found that sulfide induces phosphate release from the sludge containing iron‑phosphorus compounds (FePs) and the feasibility of recovering elemental sulfur (S⁰) during AD. To tap the full potential of sulfur in OSWs resource recovery, deciphering the sulfur transformation pathway and its influencing factors is required. Therefore, in this review, the sulfur species and distributions in OSWs and the pathway of sulfur transformation during AD were systematically summarized. Then, the relationship between iron (ferric compounds and zero-valent iron), phosphorus (FePs) and sulfur were analyzed. It was found that the reaction of iron with sulfide during AD drove the conversion of sulfide to S⁰ and iron sulfide compounds (FeS_x), and consequently iron was applied in sulfide abatement. In particular, ferric (hydr)oxide granules offer possibilities to improve the economic viability of hydrogen sulfide control by recovering S⁰. Sulfide is an interesting strategy to release phosphate from the sludge containing FePs for phosphorus recovery. Critical factors affecting sulfur transformation, including the carbon source, free ammonia and pretreatment methods, were summarized and discussed. Carbon source and free ammonia affected sulfur-related microbial diversity and enzyme activity and different sulfur transformation pathways in response to varying pretreatment methods. The study on S⁰ recovery, organic sulfur conversion, and phosphate release mechanism triggered by sulfur deserves further investigation. This review is expected to enrich our knowledge of the role of sulfur during AD and inspire new ideas for recovering phosphorus and sulfur resources from OSWs.

Enhanced anaerobic digestion of kitchen waste at different solids content by alkali pretreatment and bentonite addition: Methane production enhancement and microbial mechanism

High solid anaerobic digestion (AD) has been considered as a promising and sustainable technology for treating kitchen waste. To enhance AD of kitchen waste, alkali pretreatment and bentonite addition treatment (AP/Be) was performed on kitchen waste, and microbial community was investigated at different total solids (TS) content (10%, 13%, 19%, 22% and 25%). The results indicated that after AP/Be treatment, methane yield was as high as 198 mL CH₄/g volatile solid (VS), which increased by 236% as the control. Moreover, microbial community analysis revealed that AP/Be treatment enriched bacterial microbial diversity. At TS of 10%, AP/Be treatment enhanced the hydrogenotrophic methanogens (Methanobacterium) significantly. In addition, the dominant methanogenic pathways changed at different TS content. These results demonstrated AP/Be treatment had a positive effect on methanogenesis during kitchen waste anaerobic digestion process. This study threw new insights towards enhancing kitchen waste anaerobic digestion, as well as the microbial mechanism.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Assessment of Magnetic Nanomaterials for Municipality Wastewater Treatment Using Biochemical Methane Potential (BMP) Tests

Wastewater as a substrate potential for producing renewable energy in the form of biogas is gaining global attention. Herein, nanomaterials can be utilised as a nutrient source for microorganisms for anaerobic digestion activity. Therefore, this study explored the impact of seven different magnetic nanomaterials (MNMs) on the anaerobic digestion of wastewater via biochemical methane potential (BMP) tests for biogas production. The BMP assay was carried out with eight bioreactors, where each was charged with 50% wastewater and 30% activated sludge, leaving a headspace of 20%. Aside the control bioreactor, the other seven (7) bioreactors were dosed with 1.5 g of MNMs. This was operated under anaerobic conditions at a mesophilic temperature of 35 °C for 31 days. At the degree of 80% degradation of contaminants, the results that showed bioreactors charged with 1.5 g MNMs of TiO₂ photocatalyst composites were more effective than those constituting metallic composites, whereas the control achieved 65% degradation. Additionally, the bioreactor with magnetite (Fe₃O₄) produced the highest cumulative biogas of 1172 mL/day. Kinetically, the modified Gompertz model favoured the cumulative biogas data obtained with a significant regression coefficient (R²) close to one.

Review on fate and bioavailability of heavy metals during anaerobic digestion and composting of animal manure

Anaerobic digestion and composting are attracting increasing attention due to the increased production of animal manure. It is essential to know about the fate and bioavailability of heavy metals (HMs) for further utilisation of animal manure. This review has systematically summarised the migration of HMs and the transformation of several typical HMs (Cu, Zn, Cd, As, and Pb) during anaerobic digestion and composting. The results showed that organic matter degradation increased the HMs content in biogas residue and compost (with the exception of As in compost). HMs migrated into biogas residue during anaerobic digestion through various mechanisms. Most of HMs in biogas residue and compost exceeded relevant standards. Then, anaerobic digestion increased the bioavailable fractions proportion in Zn and Cd, decreased the F4 proportion, and raised them more than moderate environmental risks. As (III) was the main species in the digester, which extremely increased As toxicity. The increase of F3 proportion in Cu and Pb was due to sulphide formation in biogas residue. Whereas, the high humus content in compost greatly increased the F3 proportion in Cu. The F1 proportion in Zn decreased, but the plant availability of Zn in compost did not reduce significantly. Cd and As mainly converted the bioavailable fractions into stable fractions during composting, but As (V) toxicity needs to be concerned. Moreover, additives are only suitable for animal manure treated with slightly HM contaminated. Therefore, it is necessary to combine more comprehensive methods to improve the manure treatment and make product utilisation safer.

Effects of Application of Pig Manure on the Accumulation of Heavy Metals in Rice

Pig manure (PM) is often highly enriched in heavy metals, such as Cu and Zn, due to the wide use of feed additives. To study the potential risks of heavy metal accumulation in the soil and rice grains by the application of PM and other organic manure, a four-year field experiment was conducted in the suburb of Shanghai, southeast China. The contents of Cu, Zn, Pb, and Cd in the soils and rice plants by the treatments of PM and fungal culturing residues (FCR) show a trend of annual increase. Those in the soils and rice by the PM treatment are raised even more significantly. Cu and Zn contents in the soil and rice roots by the PM are significantly higher than those by the non-fertilizer control (CK) during the four years, and Pb and Cd also significantly higher than CK in the latter two years. Heavy metals taken up by the rice plants are mostly retained in the roots. Cu and Zn contents in the rice plants are in the decreasing order of roots > grains > stems > leaves, and Pb and Cd in the order of roots > stems > leaves > grains. Cu, Zn, Pb, and Cd contents in the soils by the PM treatment increase by 73%, 32%, 106%, and 127% on annual average, and those in the brown rice by 104%, 98%, 275%, and 199%, respectively. The contents of Cu, Zn, Pb, and Cd in the brown rice of the treatments are significantly correlated with those in the soils and rice roots (p < 0.05), suggesting the heavy metals accumulated in the rice grains come from the application of PM and FCR. Though the contents of heavy metals in the brown rice during the four experimental years are still within the safe levels, the risks of their accumulative increments, especially by long-term application of PM, can never be neglected.

Historical Perspective of the Addition of Magnetic Nanoparticles Into Anaerobic Digesters (2014-2021)

The addition of magnetic nanoparticles to batch anaerobic digestion was first reported in 2014. Afterwards, the number of works dealing with this subject has been increasing year by year. The discovery of the enhancement of anaerobic digestion by adding iron-based nanoparticles has created a multidisciplinary emerging research field. As a consequence, in the last years, great efforts have been made to understand the enhancement mechanisms by which magnetic nanoparticles (NPs) addition enhances the anaerobic digestion process of numerous organic wastes. Some hypotheses point to the dissolution of iron as essential iron for anaerobic digestion development, and the state of oxidation of iron NPs that can reduce organic matter to methane. The evolution and trends of this novel topic are discussed in this manuscript. Perspectives on the needed works on this topic are also presented.

Magnetite-enhanced bioelectrochemical stimulation for biodegradation and biomethane production of waste activated sludge

Microbial electrolytic cell (MEC) and magnetite (M) have shown excellent performance in promoting anaerobic digestion (AD) of biowastes. In this study, four types of anaerobic systems (i.e. single AD, M-AD, MEC-AD, and M-MEC-AD) were developed to comprehensively investigate the potential effects of magnetite-enhanced bioelectrochemical stimulation on the biodegradation of waste activated sludge (WAS) and methane (CH₄) production. Results showed that M-MEC-AD system produced the highest cumulative CH₄ yield, 9.4% higher than that observed in MEC-AD system. Bioelectrochemical stimulation enriched electroactive Geobacter, and classical methanogens (Methanosaeta and Methanobacterium), and the proliferation was further promoted when coupling with magnetite. The relative abundance of Geobacter (6.9%), Methanosaeta (0.3%), and Methanobacterium (12.6%) in M-MEC-AD system was about 10.8, 1.2, and 1.2 times of MEC-AD system, respectively. The integration of magnetite could serve as the conductive materials, and promote inherent indirect electron transfer (IET) and emerging direct electron transfer (DET) between methanogens and fermentative bacteria, building a more energy-efficient route for interspecies electron transfer and methane productivity. This study demonstrated the positive promotion of the coupled bioelectrochemical regulation and magnetite on organic biodegradation, process stability and CH₄ productivity, providing some references for the integrated technology in sludge treatment and bioenergy recovery.

Enhancing anaerobic digestion process with addition of conductive materials

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

Effect of Heavy Metals in the Performance of Anaerobic Digestion of Olive Mill Waste

This study presents an investigation on the effect of heavy metals on the production of biogas during the process of anaerobic digestion (AD) of olive mill waste (OMW). The poisonous effect and the inhibitory influence of Fe, Ni, Pb, Zn, Cu, and Cr on the digestion process are investigated and determined. Biomethanation potential tests are performed for this sake. Adding some of the heavy metals to the AD decreases the efficiency of biogas production and methane concentration and decreases the reduction in the VS, the TCOD, the SCOD, and the organic acid load. A critical increase in the total organic acid and inhibition of methanogenic bacteria was observed due to its toxicity. The toxicity of the heavy metals can be arranged according to increasing order: Cu > Ni > Pb > Cr > Zn > Fe, which leads to rapid poisoning of the active microorganisms. Iron may also exhibit stimulatory effects, but with a low rate and at a certain level. The conclusions of this work are important for the industry and help to understand how to carefully manage the presence of heavy metals in the digestate.

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).

Potential of biogas production from manure of dairy cattle fed on natural soil supplement rich in iron under batch and semi-continuous anaerobic digestion

This study provides a novel method for improving the anaerobic digestion (AD) of Holstein dairy manure (HDM) by the direct addition of Mineraso (MnS), a natural soil-derived supplement, to the feed of Holstein dairy cattle (HDC). MnS is chiefly composed of approximately 69.08% Fe₃O₄ and was supplemented at rates of 0 (F1), 25 (F2), and 50 (F3) g/head of HDC/d for two months. The HDM was then examined for non-absorbed iron prior to the batch and semi-continuous bench AD experiments. The results revealed that MnS enhanced CH₄ generation in F2 and F3 by 25% and 42%, respectively, in the batch experiments compared to that of F1. Additionally, the gas yield improved in F2 and F3 by 45% and 66%, respectively, over the control after 7 d in the bench experiments. Therefore, supplementing animals with MnS represents a sustainable and economic approach to enhancing CH₄ yields.

Prospects for biogas production and H2S control from the anaerobic digestion of cattle manure: The influence of microscale waste iron powder and iron oxide nanoparticles

Improving the quality and quantity of biogas usually requires pre-treatment to maximize methane yields and/or post-treatment to remove H₂S, which involves considerable energy consumption and higher costs. Therefore, this study proposes a cost-effective method for the enhanced anaerobic digestion (AD) of dairy manure (DM) without pre/post-treatment by directly adding waste iron powder (WIP) and iron oxide nanoparticles (INPs) to batch digesters. The results showed that the addition of iron in the form of microscale WIP (generated from the laser cutting of iron and steel) at concentrations of 100 mg/L, 500 mg/L, and 1000 mg/L improved methane yields by 36.99%, 39.36%, and 56.89%, respectively. In comparison, the equivalent dosages of INPs improved yields by 19.74%, 18.14%, and 21.11%, respectively. Additionally, the highest WIP dose (1000 mg/L) achieved the maximum improvement in the rate of hydrolysis (k), which was 1.25 times higher than in control reactions, and a maximum biomethane production rate (R_max) of 0.045 L/gVS/d according to kinetic analysis models (i.e., first-order and the Gompertz kinetic models). The rate of H₂S production was also significantly reduced (by 45.20%, 58.16%, and 77.24%) using the three WIP concentrations in comparison with INPs (which achieved reductions of 33.59%, 46.30%, and 53.52%, respectively). Therefore, the direct mixing of WIP with cattle manure is proposed as a practical and economical means of addressing complex and high-cost pre- and post-treatments that are otherwise required in the digestion process.

Effects of magnetite on anaerobic digestion of swine manure: Attention to methane production and fate of antibiotic resistance genes

Effects of magnetite on methane production and fate of antibiotic resistance genes (ARGs) during anaerobic digestion (AD) of swine manure were investigated. Results showed that methane production was increased by maximum 16.1%, and magnetite could enhance the acetoclastic methanogenesis not hydrogenotrophic methanogenesis reflected by the functional gene quantification and microbial community analysis. The propionate degradation rate was improved, and it was syntrophic oxidized into H⁺/e^-/CO₂ for direct interspecies electron transfer (DIET) and acetate, where DIET was further enhanced by magnetite and the acetate was transformed into methane through syntrophic acetate oxidation (SAO) pathway. Magnetite mainly influenced the ARGs at the interim period of AD, where ARGs especially ermF were significantly enriched. Magnetite did not influence the total ARGs abundance at the end, although the tetM was enriched and mefA was reduced finally. Statistical analysis indicated that magnetite influenced the ARGs fate mainly through the changes of microbial community.

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.

Effect of rainwater-borne hydrogen peroxide on manure-derived Cu and Zn speciation distribution and bioavailability in rice-soil system

Pot experiment was conducted to examine the effects of rainwater-borne hydrogen peroxide (H₂O₂) on transformation of Cu, Zn from pig manure in soils and its resulting impacts on the growth of Cu and Zn uptake by a rice plant. Results showed that the exogenous application of H₂O₂ significantly improved the rice biomass and yield. Addition of H₂O₂ into the soils led to reduced uptake of soil-borne Cu and Zn by the rice plants and this had a significant effect on reducing the accumulation of Zn in rice grains. It was indicated that the increased pH in soil might play important role in reducing Cu and Zn content in rice. Furthermore, Cu and Zn content in exchangeable form and carbonate bounded form dramatically decreased in soil, on the contrary, the organic combination state increased significantly in H₂O₂ treatment. The findings point to a potential research direction that rainwater-borne H₂O₂ in nature may help to change morphology of heavy metals in natural soil environments, but further study is still needed to explore the related mechanisms in Cu and Zn in manures and paddy rice field receiving rainwater-borne H₂O₂.

Effect of applied voltage and temperature on methane production and microbial community in microbial electrochemical anaerobic digestion systems treating swine manure

Microbial electrochemical technology (MET) that can harvest electricity/valuable materials and enhance the efficiency of conventional biological processes through the redox reaction of organic/inorganic compounds has attracted considerable attention. MET-based anaerobic digestion (AD) systems treating swine manure were operated at different applied voltages (0.1, 0.3, 0.5, 0.7, and 0.9 V) and temperatures (25, 35, and 45 °C). Among the MET-based AD systems with different applied voltages at 35 °C, M4 at 0.7 V showed the highest methane production (2.96 m3-CH4/m3) and methane yield (0.64 m3-CH4/kg-VS). The methane production and yield increased with increasing temperature at an applied voltage of 0.7 V. Nevertheless, the MET-based AD systems (LM at 25 °C and 0.7V) showed competitive AD performance (2.33 m3-CH4/m3 and 0.53 m3-CH4/VS) compared with the conventional AD system (35 °C). The microbial community was affected by the applied voltage and temperature, and hydrogenotrophic methanogens such as M. flavescens, M. hungatei, and M. thermautotrophicus were mainly responsible for methane production in MET-based AD systems. Therefore, the methane production can be enhanced by an applied voltage or by direct interspecies electron transfer because M. flavescens and M. thermautotrophicus were especially predominant in cathode of MET-based AD systems. The MET-based AD systems can help enhance biogas production from swine manure with no significant change in methane content. Furthermore, MET-based AD systems will be a promising AD system through low material development and the optimal operation.

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.

Biogas production and metal passivation analysis during anaerobic digestion of pig manure: effects of a magnetic Fe3O4/FA composite supplement

Anaerobic digestion has been widely used to produce biogas renewable energy and stabilize fecal manure. In this work, magnetic fly ash composites (Fe3O4/FA) were synthesized and mixed with pig manure in different ratios to study their effects on biogas production and metal passivation during anaerobic digestion. The results showed that the use of 0.5% Fe3O4/FA presented the most positive impact on biogas production compared to anaerobic digestion without Fe3O4/FA, i.e., the total biogas and methane content increased by 13.81% and 35.13%, respectively. Variations in the concentration and speciation of heavy metals (i.e., Cu and Zn) with and without Fe3O4/FA during anaerobic digestion were also analyzed. The concentrations of Cu and Zn increased after anaerobic digestion, showing a significant "relative concentration effect". Additionally, sequential fractionation suggested that Cu was mainly present in organic matter, whereas Zn was mainly distributed in the oxidation states of iron and manganese. The addition of Fe3O4/FA enhanced the passivation of Cu and Zn in the solid digested residues, i.e., the residual states of Cu and Zn increased by 10.73% to 45.78% and 33.49% to 42.14% compared to the control, respectively. Moreover, better performance was found for the treatment with 2.5% Fe3O4/FA. X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analysis demonstrated that Fe3O4/FA deactivated heavy metals mainly via physical adsorption during anaerobic digestion, which can convert them into stable mineral precipitates and thus decrease the solubility and mobility of these metals.

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.