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

Enhancement of anaerobic treatment of antibiotic pharmaceutical wastewater through the development of iron-based and carbon-based materials: A critical review

The extensive use of antibiotics has created an urgent need to address antibiotic wastewater treatment, posing significant challenges for environmental protection and public health. Recent advances in the efficacy and mechanisms of conductive materials (CMs) for enhancing the anaerobic biological treatment of antibiotic pharmaceutical wastewater are reviewed. For the first time, the focus is on the various application forms of iron-based and carbon-based CMs in strengthening the anaerobic methanogenic system. This includes the use of single CMs such as zero-valent iron (ZVI), magnetite, biochar (BC), activated carbon (AC), and graphene (GP), as well as iron-based and carbon-based composite CMs with diverse structures. These structures include mixed, surface-loaded, and core-shell combinations, reflecting the development of CMs. Iron-based and carbon-based CMs promote the rapid removal of antibiotics through adsorption and enhanced biodegradation. They also mitigate the inhibitory effects of toxic pollutants on microbial activity and reduce the expression of antibiotic resistance genes (ARGs). Additionally, as effective electron carriers, these CMs enrich microorganisms with direct interspecies electron transfer (DIET) functions, accelerate interspecies electron transfer, and facilitate the conversion of organic matter into methane. Finally, this review proposes the use of advanced molecular detection technologies to clarify microbial ecology and metabolic mechanisms, along with microscopic characterization techniques for the modification of CMs. These methods can provide more direct evidence to analyze the mechanisms underlying the cooperative anaerobic treatment of refractory organic wastewater by CMs and microorganisms.

Optimizing the management of aerobic composting for antibiotic resistance genes elimination: A review of future strategy for livestock manure resource utilization

Aerobic composting technology is an efficient, safe and practical method to reduce the residues of antibiotics and antibiotic resistance genes (ARGs) due to unreasonable disposal of livestock manure. Nowadays, it remains unclear how aerobic composting works to minimize the level of remaining antibiotics and ARGs in manure. Moreover, aerobic composting techniques even have the potential to enhance ARGs level. Therefore, this study conducted a literature review on ARGs variation during the composting process to assess the fate, migration, and risk features of antibiotics and ARGs in different livestock manure and compost. The relationship between ARGs reduction and crucial factors (temperature, heavy metal, and microbial community structures) in the composting process was discussed. The merits and limitations of different technologies used in compost was summarized. The effects on ARGs reduction in the aerobic composting process with various strategies was examined. We attempt to provide a fresh and novel viewpoint on the advancement of global aerobic composting technology.

Mitigated dissemination of antibiotic resistance genes by nanoscale zero-valent iron and iron oxides during anaerobic digestion: Roles of microbial succession and regulation

Nanoscale zero-valent iron (ZVI) and the oxides have been documented as an effective approach for mitigating the dissemination of antibiotic resistance genes (ARGs) during anaerobic digestion (AD). However, the mechanism of ARGs dissemination mitigated by nanoscale ZVI and iron oxides remain unclear. Here, we investigated the influencing mechanisms of nanoscale ZVI and iron oxides on ARGs dissemination during AD. qPCR results indicated that nanoscale ZVI and iron oxides significantly declined the total ARGs abundances, and the strongest inhibiting effect was observed by 10 g/L nanoscale ZVI. Mantel test showed ARGs distribution was positively correlated with physiochemical properties, integrons and microbial community, among which microbial community primarily contributed to ARGs dissemination (39.74%). Furthermore, redundancy and null model analyses suggested the dominant and potential ARGs host was Fastidiosipila, and homogeneous selection in the determinism factors was the largest factor for driving Fastidiosipila variation, confirming the inhibition of Fastidiosipila was primary reason for mitigating ARGs dissemination by nanoscale ZVI and iron oxides. These results were related to the inhibition of ARGs transfer related functions. This work provides novel evidence for mitigating ARGs dissemination through regulating microbial succession and regulation induced by ZVI and iron oxides.

An overview of the occurrence, impact of process parameters, and the fate of antibiotic resistance genes during anaerobic digestion processes

Antibiotic resistance genes (ARGs) have emerged as a significant global health threat, contributing to fatalities worldwide. Wastewater treatment plants (WWTPs) and livestock farms serve as primary reservoirs for these genes due to the limited efficacy of existing treatment methods and microbial adaptation to environmental stressors. Anaerobic digestion (AD) stands as a prevalent biological treatment for managing sewage sludge and manure in these settings. Given the agricultural utility of AD digestate as biofertilizers, understanding ARGs' fate within AD processes is essential to devise effective mitigation strategies. However, understanding the impact of various factors on ARGs occurrence, dissemination, and fate remains limited. This review article explores various AD treatment parameters and correlates to various resistance mechanisms and hotspots of ARGs in the environment. It further evaluates the dissemination and occurrence of ARGs in AD feedstocks and provides a comprehensive understanding of the fate of ARGs in AD systems. This review explores the influence of key AD parameters such as feedstock properties, pretreatments, additives, and operational strategies on ARGs. Results show that properties such as high solid content and optimum co-digestion ratios can enhance ARG removal, while the presence of heavy metals, microplastics, and antibiotics could elevate ARG abundance. Also, operational enhancements, such as employing two-stage digestion, have shown promise in improving ARG removal. However, certain pretreatment methods, like thermal hydrolysis, may exhibit a rebounding effect on ARG levels. Overall, this review systematically addresses current challenges and offers future perspectives associated with the fate of ARGs in AD systems.

The role of water matrix on antibiotic resistance genes transmission in substrate layer from stormwater bioretention cells

Recently, extensive attention has been paid to antibiotic resistance genes (ARGs) transmission. However, little available literature could be found about ARGs transmission in stormwater bioretention cells, especially the role of water matrix on ARGs transmission. Batch experiments were conducted to investigate target ARGs (blaTEM, tetR and aphA) transmission behaviors in substrate layer from stormwater bioretention cells under different water matrices, including nutrient elements (e.g., carbon, nitrogen and phosphorus), water environmental conditions (dissolved oxygen (DO), pH and salinity, etc.) and pollution factors (like heavy metals, antibiotics and disinfectants), showing that ARGs conjugation frequency increased sharply with the enhancement of water matrices (expect DO and pH), while there were obvious increasing tendencies for all ARGs transformation frequencies under only the pollution factor. The correlation between dominant bacteria and ARGs transmission implied that conjugation and transformation of ARGs were mainly determined by Firmicutes, Bacteroidota, Latescibacterota, Chloroflexi and Cyanobacteria at the phylum level, and by Sphingomonas, Ensifer, IMCC26256, Rubellimicrobium, Saccharimonadales, Vicinamibacteraceae, Nocardioides, JG30-KF-CM66 at the genus level. The mentioned dominant bacteria were responsible for intracellular reactive oxygen species (ROS) and cell membrane permeability (CMP) in the substrate layer, where the amplification of intracellular ROS variation were the largest with 144 and 147 % under the condition of TP and salinity, respectively, and the one of CMP variation were the highest more than 165 % under various pollution factors. Furthermore, both increasing DO and reducing salinity could be potential approaches for the inhibition of ARGs transmission in bioretention cells taking into account the simultaneous removal of conventional pollutants.

Impacts of electric field-magnetic powder coupled membrane bioreactor on phenol wastewater treatment: Performance, synergistic mechanism, antibiotic resistance genes, and eco-environmental benefit evaluation

A novel electric field-magnetic powder coupled membrane bioreactor (EM-MBR) was constructed, which was superior on improvement of phenol treatment performance and sludge characteristics, and mitigation of membrane fouling. EM-MBR enhanced the phenol degradation via the improvement activity of phenol degrading enzymes. The EPS contents and SVI of EM-MBR were significantly reduced by 49.3 % and 58.7 % than that of the conventional MBR, respectively. Moreover, EM-MBR successfully reduced fouling rate by 57.0 %, delaying the membrane resistance. The EPS contents were positively correlated with the SVI and fouling rate, implying that the sludge settleability was strengthened by improving the properties of EPS with the assistance of electromagnetic, thus mitigating the membrane fouling. Microbial co-occurrence network demonstrated that EM-MBR enriched phenol-degrading and EPS-degrading genera correlated to Fe redox cycle. Furthermore, the activation of the antioxidant system in the EM-MBR resulted in the suppression of reactive oxygen species (ROS) generation, consequently impeding the dissemination of antibiotic resistance genes (ARGs). Co-occurrence patterns of MGEs and ARGs revealed that intercellular binding facilitated by ist and Integrase may account for the horizontal transfer of ARGs. The reduction of unit capital costs (15.63 %), running costs (53.00 %), and total average carbon emissions (15.18 %) indicated that EM-MBR was environmentally beneficial.

Disturbance mitigation of thiencarbazone-methyl·isoxaflutole on bacterial communities through nitrification inhibitor and attapulgite

There is a knowledge gap in the interaction between the effects of herbicide thiencarbazone-methyl·isoxaflutole on soil microflora and environmental parameters, which leads to a lack of measures in mitigating damage to bacterial communities from the herbicide use. The impacts of thiencarbazone-methyl·isoxaflutole and soil parameters on the diversity, structure and functions of soil bacterial communities were clarified, and the effects and potential mitigation mechanisms of nitrapyrin and modified attapulgite with bacterial function intervention on bacterial communities were explored through incubation and field experiments. The results showed that as thiencarbazone-methyl·isoxaflutole application increased, the stress on soil bacterial community structure and diversity also increased. The relative abundance of bacteria including Aridibacter and GP7 and functional annotations including "nitrate_reduction" were significantly negatively correlated with thiencarbazone-methyl·isoxaflutole residues in soils. The remarkable toxic effects on the Adhaeribacter bacteria were detected at the recommended dose of thiencarbazone-methyl·isoxaflutole application. The residue of isoxaflutole (one of the effective ingredients of thiencarbazone-methyl·isoxaflutole) directly and more strongly affected the diversity of soil bacterial communities than thiencarbazone-methyl. Increasing soil pH was recognised as an important factor in improving the diversity and structure of soil microflora based on the results of the Mantel test and canonical correspondence analysis. Supplemental use of nitrapyrin or modified attapulgite was found to increase soil pH, and further improve the expression of "manganese oxidation" function annotation. This contributed to the increased bacterial diversity (Shannon index). Therefore, the disturbance of soil microflora caused by thiencarbazone-methyl·isoxaflutole application can be mitigated by the use of nitrapyrin and modified attapulgite through raising soil pH.

Carbamazepine facilitated horizontal transfer of antibiotic resistance genes by enhancing microbial communication and aggregation

Antimicrobial resistance is a global health security issue of widespread concern. Recent studies have unveiled the potential contribution of non-antibiotics to the emergence of antimicrobial resistance. This study investigated the effect of carbamazepine, a non-antibiotic pharmaceutical, on the fate of antibiotic resistance genes (ARGs) during anaerobic digestion. The results, as revealed by both metagenomic sequencing and absolute quantification, demonstrated that carbamazepine induced the enrichment of ARGs and increased the abundance of ARGs hosts by 1.2-2.1 times. Carbamazepine facilitated microbial aggregation and intercellular communication by upregulating functional genes associated with two-component systems, quorum sensing and type IV secretion systems, thereby increasing the frequency of ARGs conjugation. Furthermore, carbamazepine induced the acquisition of ARGs by pathogens and elevated the overall pathogenic abundance. This study revealed the mechanisms of microbial self-regulation and ARGs transmission under carbamazepine stress, highlighting the potential health risks posed by non-antibiotic pharmaceuticals during the safe disposal of sludge.

Nanotechnology based technological development in biofuel production: Current status and future prospects

Depleting fossil fuels and net carbon emissions associated with their burning have driven the need to find alternative energy sources. Biofuels are near-perfect candidates for alternative energy sources as they are renewable and account for no net CO2 emissions. However, biofuel production must overcome various challenges to compete with conventional fuels. Conventional methods for bioconversion of biomass to biofuel include chemical, thermochemical, and biological processes. Substrate selection and processing, low yield, and total cost of production are some of the main issues associated with biofuel generation. Recently, the uses of nanotechnology and nanoparticles have been explored to improve the biofuel production processes because of their high adsorption, high reactivity, and catalytic properties. The role of these nanoscale particles and nanocatalysts in biomass conversion and their effect on biofuel production processes and yield are discussed in the present article. The applicability of nanotechnology in production processes of biobutanol, bioethanol, biodiesel, biohydrogen, and biogas under biorefinery approach are presented. Different types of nanoparticles, and their function in the bioprocess, such as electron transfer, pretreatment, hydrolysis, microalgae cultivation, lipid extraction, dark and photo fermentation, immobilization, and suppression of inhibitory compounds, are also highlighted. Finally, the current and potential applications of nanotechnology in biorefineries are also discussed.

Effects of iron additives on the removal of antibiotics and antibiotic resistance genes in anaerobic fermentation of food waste

The presence of antibiotics and antibiotic resistance genes (ARGs) in food waste (FW) during anaerobic fermentation poses significant environmental and health risks. This study elucidated the potential of iron additives, specifically 500-nm and 50-nm zero-valent iron (ZVI) and magnetite, in mitigating these contaminants. These findings revealed that 500-nm magnetite significantly reduced tetracyclines by 81.04%, while 500-nm ZVI effectively reduced cefotaxime by 89.90%. Furthermore, both 500-nm and 50-nm ZVI were observed to decrease different types and abundance of heavy metal resistance and virulence genes. Interestingly, while 500-nm ZVI reduced the overall abundance of ARGs by 50%, 500-nm magnetite primarily reduced the diversity of ARGs without significantly impacting their abundance. These results elucidate the efficacy of iron additives in addressing antibiotic contamination and resistance during the anaerobic fermentation process of FW. The findings acquired from this study mitigate the development of innovative and environmentally sustainable technologies for FW treatment, emphasizing the reduction of environmental risks and enhancement of treatment efficiency.

Exploring carbon conversion and balance with magnetite-amended during pig manure composting

This study was designed to explore the magnetite in maturation and humification during pig manure (PM) and wolfberry branch fillings (BF) composting. Different proportions of magnetite (T1, 0%; T2, 2.5%; T3, 5%; T4, 7.5%;) were blended with PM for 50 days of composting. The findings indicated magnetite amendment has no influence on the maturity, and the 5% ratio significantly promoted humic acid (HA) formation and fulvic acid (FA) decomposition compared to other treatments. Compared to T1, magnetite addition significantly increased CO2 and CH4 emissions by 106.39%-191.69% and 6.88-13.72 times. The further analysis suggested that magnetite improved Ruminofilibacter activity were significantly positively associated with HA, and C emissions. The further PICRUSt 2 analysis showed membrane transport may enhance environmental information processing by magnetite. Overall, these results demonstrated higher organic matter (OM) degradation and HA formation with an additional increase in microbial activity highlighted advantages of using magnetite during PM composting.

Risk control of antibiotics, antibiotic resistance genes (ARGs) and antibiotic resistant bacteria (ARB) during sewage sludge treatment and disposal: A review

Sewage sludge is an important reservoir of antibiotics, antibiotic resistance genes (ARGs), and antibiotic resistant bacteria (ARB) in wastewater treatment plants (WWTPs), and the reclamation of sewage sludge potentially threats human health and environmental safety. Sludge treatment and disposal are expected to control these risks, and this review summarizes the fate and controlling efficiency of antibiotics, ARGs, and ARB in sludge involved in different processes, i.e., disintegration, anaerobic digestion, aerobic composting, drying, pyrolysis, constructed wetland, and land application. Additionally, the analysis and characterization methods of antibiotics, ARGs, and ARB in complicate sludge are reviewed, and the quantitative risk assessment approaches involved in land application are comprehensively discussed. This review benefits process optimization of sludge treatment and disposal, with regard to environmental risks control of antibiotics, ARGs, and ARB in sludge. Furthermore, current research limitations and gaps, e.g., the antibiotic resistance risk assessment in sludge-amended soil, are proposed to advance the future studies.

Magnetic biochar promotes the risk of mobile genetic elements propagation in sludge anaerobic digestion

Mobile genetic elements (MGEs) mediated horizontal gene transfer is the primary reason for the propagation of antibiotic resistance genes in environment. The behavior of MGEs under magnetic biochar pressure in sludge anaerobic digestion (AD) is still unknown. This study evaluated the effects of different dosage magnetic biochar on the MGEs in AD reactors. The results showed that the biogas yield was highest (106.68 ± 1.16 mL g^-1 VS_added) with adding optimal dosage of magnetic biochar (25 mg g^-1 TS_added), due to it increased the microorganism's abundance involved in hydrolysis and methanogenesis. While, the total absolute abundance of MGEs in the reactors with magnetic biochar addition increased by 11.58%-77.37% compared with the blank reactor. When the dosage of magnetic biochar was 12.5 mg g^-1 TS_added, the relative abundance of most MGEs was the highest. The enrichment effect on ISCR1 was the most significant, and the enrichment rate reached 158.90-214.16%. Only the intI1 abundance was reduced and the removal rates yield 14.38-40.00%, which was inversely proportional to the dosage of magnetic biochar. Co-occurrence network explored that Proteobacteria (35.64%), Firmicutes (19.80%) and Actinobacteriota (15.84%) were the main potential host of MGEs. Magnetic biochar changed MGEs abundance by affecting the potential MGEs-host community structure and abundance. Redundancy analysis and variation partitioning analysis showed that the combined effect of polysaccharides, protein and sCOD exhibited the greatest contribution (accounted for 34.08%) on MGEs variation. These findings demonstrated that magnetic biochar increases the risk of MGEs proliferation in AD system.

A review of the impact of conductive materials on antibiotic resistance genes during the anaerobic digestion of sewage sludge and animal manure

The urgent need to reduce the environmental burden of antibiotic resistance genes (ARGs) has become even more apparent as concerted efforts are made globally to tackle the dissemination of antimicrobial resistance. Concerning levels of ARGs abound in sewage sludge and animal manure, and their inadequate attenuation during conventional anaerobic digestion (AD) compromises the safety of the digestate, a nutrient-rich by-product of AD commonly recycled to agricultural land for improvement of soil quality. Exogenous ARGs introduced into the natural environment via the land application of digestate can be transferred from innocuous environmental bacteria to clinically relevant bacteria by horizontal gene transfer (HGT) and may eventually reach humans through food, water, and air. This review, therefore, discusses the prospects of using carbon- and iron-based conductive materials (CMs) as additives to mitigate the proliferation of ARGs during the AD of sewage sludge and animal manure. The review spotlights the core mechanisms underpinning the influence of CMs on the resistome profile, the steps to maximize ARG attenuation using CMs, and the current knowledge gaps. Data and information gathered indicate that CMs can profoundly reduce the abundance of ARGs in the digestate by easing selective pressure on ARGs, altering microbial community structure, and diminishing HGT.

Composting pig manure with nano-zero-valent iron amendment: Insights into the carbon cycle and balance

The effectiveness of nano-zero-valent-iron (NZVI) addition during composting of pig manure (PM) was investigated. Different dosages of NZVI were mixed with PM substrate during a 50 days composting process. The results revealed that the higher share of NZVI addition, the higher OM degradation rate is. On contrary, it was observed that the higher share of NZVI addition, the lower the fulvic acid and the humin degradation rate is. Meanwhile, NZVI amendment increased the CO₂ and CH₄ emissions by 29-47 % and 53-57 %, respectively. The in-depth analysis showed that NZVI addition increased the activity of Sphaerobacter and Luteimonas, which eventually led to the degradation of hard-to-degrade OM faster. Additionally, NZVI was found to increase the filtration of microorganisms, reducing the toxicity and hygiene of compost products. No significant improvement in humic substance enhancement was observed during composting with NZVI addition but improved OM degradation.

Effects of Magnetic Biochar Addition on Mesophilic Anaerobic Digestion of Sewage Sludge

As a low-cost additive to anaerobic digestion (AD), magnetic biochar (MBC) can act as an electron conductor to promote electron transfer to enhance biogas production performance in the AD process of sewage sludge and has thus attracted much attention in research and industrial applications. In the present work, Camellia oleifera shell (COS) was used to produce MBC as an additive for mesophilic AD of sewage sludge, in order to explore the effect of MBC on the mesophilic AD process and its enhancement mechanism. Analysis by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectrometry (FTIR), and X-ray diffraction (XRD) further confirmed that biochar was successfully magnetized. The yield of biogas from sewage sludge was enhanced by 14.68-39.24% with the addition of MBC, and the removal efficiency of total solid (TS), volatile solids (VS), and soluble chemical oxygen demand (sCOD) were 28.99-46.13%, 32.22-48.62%, and 84.18-86.71%, respectively. According to the Modified Gompertz Model and Cone Model, the optimum dosage of MBC was 20 mg/g TS. The maximum methane production rate (R_m) was 15.58% higher than that of the control reactor, while the lag-phase (λ) was 43.78% shorter than the control group. The concentration of soluble Fe²⁺ and Fe³⁺ were also detected in this study to analyze the function of MBC for improving biogas production performance from sewage sludge. The biogas production was increased when soluble Fe³⁺ was reduced to soluble Fe²⁺. Overall, the MBC was beneficial to the resource utilization of COS and showed a good prospect for improving mesophilic AD performance.

Biofouling control potential of quorum quenching anaerobes in lab-scale anaerobic membrane bioreactors: Foulants profile and microbial dynamics

Indigenously isolated anaerobes encoding four quorum quenching (QQ) enzymes were applied in immobilized- and bioaugmented forms for their implications on membrane foulants, microbial taxa, and biofouling control. Two identical anaerobic membrane bioreactors (AnMBRs) with different immobilizing media, i.e. silica-alginate (AnMBR-Si) and hollow fiber-alginate (AnMBR-Hf), were sequentially operated for two conventional and three QQ based phases. The synergistic addition of QQ anaerobes in free cells and the immobilized form prolonged the membrane filtration operation by 172 ± 29% and 284 ± 12% in AnMBR-Si and AnMBR-Hf, respectively. Biocake with low surface coverage was prominent during QQ application compared to conventional phases. Despite the better control of AHLs (3OC6-, C6-, 3OC8, C8, and C10-HSL) and AI-2 at various points of QQ phases, the QQ consortium could not maintain a low concentration of signals for longer period. Therefrom, quenching of targeted signal molecules instigate the dominance of microbial species bearing non-targeted quorum sensing mechanism. The QQ significantly altered the biofilm-forming community in mixed liquor, while the members with robust signal transduction systems became dominant to counteract the QQ mechanism and were the ultimate cause of biofouling. The improved methane content in biogas and increased methanogens composition during QQ phases demonstrated the synergism of exogenous and immobilized QQ as the most viable option for long-term AnMBR operation.

Transmission of antibiotic resistance genes through mobile genetic elements in Acinetobacter baumannii and gene-transfer prevention

Antibiotic resistance is a major global public health concern. Acinetobacter baumannii is a nosocomial pathogen that has emerged as a global threat because of its high levels of resistance to many antibiotics, particularly those considered as last-resort antibiotics, such as carbapenems. Mobile genetic elements (MGEs) play an important role in the dissemination and expression of antibiotic resistance genes (ARGs), including the mobilization of ARGs within and between species. We conducted an in-depth, systematic investigation of the occurrence and dissemination of ARGs associated with MGEs in A. baumannii. We focused on a cross-sectoral approach that integrates humans, animals, and environments. Four strategies for the prevention of ARG dissemination through MGEs have been discussed: prevention of airborne transmission of ARGs using semi-permeable membrane-covered thermophilic composting; application of nanomaterials for the removal of emerging pollutants (antibiotics) and pathogens; tertiary treatment technologies for controlling ARGs and MGEs in wastewater treatment plants; and the removal of ARGs by advanced oxidation techniques. This review contemplates and evaluates the major drivers involved in the transmission of ARGs from the cross-sectoral perspective and ARG-transfer prevention processes.

Effect of biochar on antibiotic resistance genes in the anaerobic digestion system of antibiotic mycelial dreg

To address the problem of antibiotic mycelial dreg (AMD) treatment and removal of antibiotic resistance genes (ARGs), this study adopted anaerobic digestion (AD) technology, and added biochar (BC) and biochar loaded with nanosized zero-valent iron (nZVI-BC) to promote the AD of AMD and enhance the removal of ARGs. Results showed that nZVI-BC was better than BC in promoting AD due to the hydrogen evolution corrosion and the synergistic effect of nZVI and BC. In addition, BC and nZVI-BC can enhance the oxidative stress response and reduce ammonia stress phenomenon, which significantly reduces the abundance of aadA, ant(2″)-Ⅰ, qacEdelta1 and sul1. In conclusion, the enhance effect of nZVI-BC is greater than BC. The removal efficiency rates of nZVI-BC on the above-mentioned four ARGs were improved by 33%, 9%, 24% and 11%.

The role of iron nanoparticles on anaerobic digestion: mechanisms, limitations, and perspectives

Anaerobic digestion (AD) is the most widely used technology for organic matter treatment. However, multiple types of research have reported on improving the process because different operation inhibition factors and limitations affect the performance of AD process. Owing to the increasing use of iron-nanoparticles (Fe-NP) on AD, this review addresses the knowledge gaps and summarizes the finding from academic articles based on (i) the AD upgrading operations: limitations and upgrade techniques, (ii) Fe-NPs mechanisms on AD, (iii) Fe-NP effect on microbial communities associated to AD systems, and (iv) perspectives. The selected topics give the Fe-NP positive effects on the AD methane-production process in terms of gas production, effluent quality, and process optimization. The main results of this work indicate that (i) Fe-NP addition can be adapted among different feedstocks and complement other pretreatments, (ii) Fe-NP physicochemical characteristics enhance biogas production via direct interspecies electron transfer (DIET) mechanisms, and Fe-ion release due to their structure and their conductivity capability, and (iii) syntrophic bacteria and acetoclastic methanogens have been reported as the communities that better uptake Fe-NPs on their metabolisms. Finally, our research perspectives and gaps will be discussed to contribute to our knowledge of using Fe-NPs on AD systems.

Insight into effects of polyethylene microplastics in anaerobic digestion systems of waste activated sludge: Interactions of digestion performance, microbial communities and antibiotic resistance genes

The environmental risks of microplastics (MPs) have raised an increasing concern. However, the effects of MPs in anaerobic digestion (AD) systems of waste activated sludge (WAS), especially on the fate of antibiotic resistance genes (ARGs), have not been clearly understood. Herein, the variation and interaction of digestion performance, microbial communities and ARGs during AD process of WAS in the presence of polyethylene (PE) MPs with two sizes, PE MPs-180μm and PE MPs-1mm, were investigated. The results showed that the presence of PE MPs, especially PE MPs-1mm, led to the increased hydrolysis of soluble polysaccharides and proteins and the accumulation of volatile fatty acids. The methane production decreased by 6.1% and 13.8% in the presence of PE MPs-180μm and PE MPs-1mm, respectively. Together with this process, hydrolytic bacteria and acidogens were enriched, and methanogens participating in acetoclastic methanogenesis were reduced. Meanwhile, ARGs were enriched obviously by the presence of PE MPs, the abundances of which in PE MPs-180μm and PE MPs-1mm groups were 1.2-3.0 times and 1.5-4.0 times higher than that in the control by the end of AD. That was associated with different co-occurrence patterns between ARGs and bacterial taxa and the enrichment of ARG-hosting bacteria caused by the presence of PE MPs. Together these results suggested the adverse effects of PE MPs on performance and ARGs removal during AD process of WAS through inducing the changes of microbial populations.

Uncovering the effect of polyethyleneimine on methane production in anaerobic digestion of sewage sludge

The potential effect of polyethyleneimine as a flocculant on anaerobic digestion of sludge was investigated. Polyethyleneimine above 12 g/kg total suspended solids inhibited the entire anaerobic digestion process including solubilization, hydrolysis, acidification, and methanogenesis. The addition of 24 g/kg total suspended solids polyethyleneimine reduced methane production from 167 ± 5 L/kg volatile suspended solids in the control reactor (without polyethyleneimine) to 141 ± 5 L/kg volatile suspended solids. Polyethyleneimine bound to extracellular polymeric substances, thus enhancing sludge agglomeration and hindering the release of organics. Meanwhile, the reduction of cytochrome C impeded electron transport, consequently curbed direct interspecies electron transfer. The adsorption of carbon dioxide by amine groups also hampered methane conversion. This study elucidated the concept that polyethyleneimine reduces mass transfer in anaerobic digestion, providing new insights into the potential behavior of flocculants in sludge treatment.

Dissecting the roles of conductive materials in attenuating antibiotic resistance genes: Evolution of physiological features and bacterial community

Supplying conductive materials (CMs) into anaerobic bioreactors is considered as a promising technology for antibiotic wastewater treatment. However, whether and how CMs influence antibiotic resistance genes (ARGs) spread remains poorly known. Here, we investigated the effects of three CMs, i.e., magnetite, activated carbon (AC), and zero valent iron (ZVI), on ARGs dissemination during treating sulfamethoxazole wastewater, by dissecting the shifts of physiological features and microbial community. With the addition of magnetite, AC, and ZVI, the SMX removal was improved from 49.05 to 71.56-92.27 %, while the absolute abundance of ARGs reducing by 26.48 %, 61.95 %, 48.45 %, respectively. The reduced mobile genetic elements and antibiotic resistant bacteria suggested the inhibition of horizontal and vertical transfer of ARGs. The physiological features, including oxidative stress response, quorum sensing, and secretion system may regulate horizontal transfer of ARGs. The addition of all CMs relieved oxidative stress compared with no CMs, but ZVI may cause additional free radicals that needs to be concerned. Further, ZVI and AC also interfered with cell communication and secretion system. This research deepens the insights about the underlying mechanisms of CMs in regulating ARGs, and is expected to propose practical ways for mitigating ARGs proliferation.

Electron shuttles enhanced the removal of antibiotics and antibiotic resistance genes in anaerobic systems: A review

The environmental and epidemiological problems caused by antibiotics and antibiotic resistance genes have attracted a lot of attention. The use of electron shuttles based on enhanced extracellular electron transfer for anaerobic biological treatment to remove widespread antibiotics and antibiotic resistance genes efficiently from wastewater or organic solid waste is a promising technology. This paper reviewed the development of electron shuttles, described the mechanism of action of different electron shuttles and the application of enhanced anaerobic biotreatment with electron shuttles for the removal of antibiotics and related genes. Finally, we discussed the current issues and possible future directions of electron shuttle technology.

Nanomaterials for biogas augmentation towards renewable and sustainable energy production: A critical review

Nanotechnology is considered one of the most significant advancements in science and technology over the last few decades. However, the contemporary use of nanomaterials in bioenergy production is very deficient. This study evaluates the application of nanomaterials for biogas production from different kinds of waste. A state-of-the-art comprehensive review is carried out to elaborate on the deployment of different categories of nano-additives (metal oxides, zero-valent metals, various compounds, carbon-based nanomaterials, nano-composites, and nano-ash) in several kinds of biodegradable waste, including cattle manure, wastewater sludge, municipal solid waste, lake sediments, and sanitary landfills. This study discusses the pros and cons of nano-additives on biogas production from the anaerobic digestion process. Several all-inclusive tables are presented to appraise the literature on different nanomaterials used for biogas production from biomass. Future perspectives to increase biogas production via nano-additives are presented, and the conclusion is drawn on the productivity of biogas based on various nanomaterials. A qualitative review of relevant literature published in the last 50 years is conducted using the bibliometric technique for the first time in literature. About 14,000 research articles are included in this analysis, indexed on the Web of Science. The analysis revealed that the last decade (2010–20) was the golden era for biogas literature, as 84.4% of total publications were published in this timeline. Moreover, it was observed that nanomaterials had revolutionized the field of anaerobic digestion, methane production, and waste activated sludge; and are currently the central pivot of the research community. The toxicity of nanomaterials adversely affects anaerobic bacteria; therefore, using bioactive nanomaterials is emerging as the best alternative. Conducting optimization studies by varying substrate and nanomaterials’ size, concentration and shape is still a field. Furthermore, collecting and disposing nanomaterials at the end of the anaerobic process is a critical environmental challenge to technology implementation that needs to be addressed before the nanomaterials assisted anaerobic process could pave its path to the large-scale industrial sector.

Potential of biogas residue biochar modified by ferric chloride for the enhancement of anaerobic digestion of food waste

Biogas residue biochar (BRB) and BRB modified by ferric chloride (BRB-FeCl₃) were applied to promote anaerobic digestion (AD) of food waste (FW), related mechanisms were also proposed in this study. Results indicated BRB-FeCl₃ showed higher specific surface area, more abundant functional groups and impregnate iron than BRB, and they respectively increased 22.50% and 12.79% cumulative methane yields compared with control group because of accelerated volatile fatty acids (VFAs) transformation, which were confirmed by enhanced metabolism of glycolysis, fatty acid degradation and pyruvate. BRB, especially BRB-FeCl₃ facilitated the growth of Syntrophomonas, Methanofollis, Methanoculleus and Methanosarcina, which further promoted the methanogenesis by enhancing the metabolic activities of methanol, dimethylamine and methylamine pathways, thereby causing more metabolically diverse methanogenic pathways. Metagenomics analysis revealed BRB, especially BRB-FeCl₃ promoted the relative abundances of functional genes involved in direct interspecies electron transfer (DIET). Present study explored the enhancement mechanisms and feasibility of BRB-FeCl₃ for AD process.

Enhancement of Anaerobic Digestion with Nanomaterials: A Mini Review

In recent years, the number of articles reporting the addition of nanomaterials to enhance the process of anaerobic digestion has exponentially increased. The benefits of this addition can be observed from different aspects: an increase in biogas production, enrichment of methane in biogas, elimination of foaming problems, a more stable and robust operation, absence of inhibition problems, etc. In the literature, one of the current focuses of research on this topic is the mechanism responsible for this enhancement. In this sense, several hypotheses have been formulated, with the effect on the redox potential caused by nanoparticles probably being the most accepted, although supplementation with trace materials coming from nanomaterials and the changes in microbial populations have been also highlighted. The types of nanomaterials tested for the improvement of anaerobic digestion is today very diverse, although metallic and, especially, iron-based nanoparticles, are the most frequently used. In this paper, the abovementioned aspects are systematically reviewed. Another challenge that is treated is the lack of works reported in the continuous mode of operation, which hampers the commercial use of nanoparticles in full-scale anaerobic digesters.

Performance and metagenomics analysis of anaerobic digestion of food waste with adding biochar supported nano zero-valent iron under mesophilic and thermophilic condition

A large amount of food waste (FW) brings environmental pollution and sanitation problems. Anaerobic digestion (AD) is an effective technology to treat FW and generate biogas energy. This study investigated the effect of biochar supported nano zero-valent iron (BC-nZVI) on AD performance of FW. Results showed that the cumulative methane yield (CMY) increased by 21.52%-54.90% and the lag time decreased significantly with BC-nZVI. Under mesophilic and thermophilic condition, the peak of CMY was achieved at 178.82 ± 5.27 mL/g VS and 193.01 ± 6.81 mL/g VS with 5 g/L BC-nZVI, respectively. Besides, BC-nZVI stimulated hydrolysis process and reduced the inhibition of NH₄⁺-N and volatile fatty acids accumulation, and it could improve the system stability. Structural equation model analysis indicated that digestion time, BC-nZVI, NH₄⁺-N, temperature and total volatile fatty acid had significant effects on CMY, explaining 92.20% of its total variation. The metagenomic analysis of key microorganisms and related metabolism pathways involved in AD system was further investigated. The results suggested that BC-nZVI contributed to strengthen methanogenesis through enriching the various predominant methanogenic pathways and activating most enzymes related to methane metabolism. BC-nZVI could improve the AD system function and provided a better AD performance by shifting the microbial communities and altering functional genes. This study provided a theoretical basis for BC-nZVI applications and improvements in AD process of FW.

Insights into structure and activity relationship of clinically mutated PER1 and PER2 class A β-lactamase enzymes

PER1 and PER2 are among the class A β-lactamase enzymes, which have evolved clinically to form antibiotic resistance and have significantly expanded their spectrum of activity. Hence, there is a need to study the clinical mutation responsible for such β-lactamase mediated antibiotic resistance. Alterations in catalytic centre and Ω-loop structure could be the cause of antibiotic resistance in these β-lactamase enzymes. Structural and functional alterations are caused due to mutations on or near the catalytic centre, which results in active site plasticity and are responsible for its expanded spectrum of activity in these class A β-lactamase enzymes. Multiple sequence alignment, structure, kinetic, molecular docking, MMGBSA and molecular dynamic simulation comparisons were done on 38 clinically mutated and wild class A β-lactamase enzymes. This work shows that PER1 and PER2 enzymes contains most unique mutations and have altered Ω-loop structure, which could be responsible for altering the structure-activity relationship and extending the spectrum of activity of these enzymes. Alterations in molecular docking, MMGBSA, kinetic values reveals the modification in the binding and activity of these clinically mutated enzymes with antibiotics. Further, the cause of these alterations can be revealed by active site interactions and H-bonding pattern of these enzymes with antibiotics. Met69Gln, Glu104Thr, Tyr105Trp, Met129His, Pro167Ala, Glu168Gln, Asn170His, Ile173Asp and Asp176Gln mutations were uniquely found in PER1 and PER2 enzymes. These mutations occurs at catalytic important residues and results in altered interactions with β-lactam antibiotics. Hence, these mutations could be responsible for altering the structure-activity of PER1 and PER2 enzymes.

A meta-analysis reveals that operational parameters influence levels of antibiotic resistance genes during anaerobic digestion of animal manures

Anaerobic digestion (AD) has shown the potential to reduce the numbers and types of antibiotic-resistance genes (ARG) present in animal manures. However, the variability of the results has limited the ability to draw solid conclusions. To address this issue, we performed a series of meta-analyses to evaluate how AD of pig, cattle, and dairy manures affects ARG levels and how different parameters, such as temperature, pH, digestion times, and the addition of other substances (e.g., solids, antibiotics) influence ARG changes. Twenty studies with enough details on changes in ARG levels during the AD process were identified and used for the meta-analyses. The results suggested that AD could significantly reduce ARG levels regardless of the conditions of the process. Also, thermophilic AD was more effective than mesophilic AD at reducing ARGs, although this difference was only significant for pig manures. The results also suggested that long digestion times (>50 days) yielded better ARG reduction rates, and that the addition of solids from an external source (co-digestion) negatively affected the efficiency of ARG reduction. In general, the results suggested that ARG changes during AD could be linked to the abundance and activity of hydrolytic communities.

Antibiotic Resistance in the Drinking Water: Old and New Strategies to Remove Antibiotics, Resistant Bacteria, and Resistance Genes

Bacterial resistance is a naturally occurring process. However, bacterial antibiotic resistance has emerged as a major public health problem in recent years. The accumulation of antibiotics in the environment, including in wastewaters and drinking water, has contributed to the development of antibiotic resistant bacteria and the dissemination of antibiotic resistance genes (ARGs). Such can be justified by the growing consumption of antibiotics and their inadequate elimination. The conventional water treatments are ineffective in promoting the complete elimination of antibiotics and bacteria, mainly in removing ARGs. Therefore, ARGs can be horizontally transferred to other microorganisms within the aquatic environment, thus promoting the dissemination of antibiotic resistance. In this review, we discuss the efficiency of conventional water treatment processes in removing agents that can spread/stimulate the development of antibiotic resistance and the promising strategies for water remediation, mainly those based on nanotechnology and microalgae. Despite the potential of some of these approaches, the elimination of ARGs remains a challenge that requires further research. Moreover, the development of new processes must avoid the release of new contaminants for the environment, such as the chemicals resulting from nanomaterials synthesis, and consider the utilization of green and eco-friendly alternatives such as biogenic nanomaterials and microalgae-based technologies.

Characteristics of antibiotic resistance genes in full-scale anaerobic digesters of food waste and the effects of application of biogas slurry on soil antibiotic resistance genes

The fate of antibiotic resistance genes (ARGs) in full-scale anaerobic digestion (AD) of food waste (FW) and in the soil applied with biogas slurry has not been fully understood. In this study, 12 targeted ARGs and intI1 in FW, intermediate product, and biogas slurry from three full-scale AD were analyzed. The results showed that subcritical water pretreatment was an effective method for ARG attenuation, by which the absolute abundance of total targeted ARGs was removed by 99.69%. The predominant ARGs (ermB, tetM, and tetW) in FW were removed more than 99% after subcritical water pretreatment. The result of field experiments with biogas slurry as fertilizer showed that the absolute abundance of several ARGs (sul2, tetM, blaOXA-1, blaTEM) and intI1 accumulated significantly compared to the control group (CK) during three consecutive growth stages of the rice. The detected abundance of ARGs in paddy field soil increased from 190.50 (CK) to 8.87 × 10⁴ copies/g (wet weight) (soil) during tillering stage, and increased from 4102.65 (CK) to 4.38 × 10⁴ copies/g (wet weight) (soil) during heading time. Biogas slurry improved the soil nutrients (TN, AN, TP, and AP); meanwhile, the concentrations of total salt and Cl^- increased. Network analysis indicated that 28 genera were the possible hosts of ARGs; variation partitioning analysis (VPA) indicated that microbial communities (contribution 59.30%) were the main factors that affected the fate of ARGs and intI1.

The role of iron-based nanoparticles (Fe-NPs) on methanogenesis in anaerobic digestion (AD) performance

Several strategies have been proposed to improve the performance of the anaerobic digestion (AD) process. Among them, the use of various nanoparticles (NPs) (e.g. Fe, Ag, Cu, Mn, and metal oxides) is considered one of the most effective approaches to enhance the methanogenesis stage and biogas yield. Iron-based NPs (zero-valent iron with paramagnetic properties (Fe⁰) and iron oxides with ferromagnetic properties (Fe₃O₄/Fe₂O₃) enhance microbial activity and minimise the inhibition effect in methanogenesis. However, comprehensive and up-to-date knowledge on the function and impact of Fe-NPs on methanogens and methanogenesis stages in AD is frequently required. This review focuses on the applicative role of iron-based NPs (Fe-NPs) in the AD methanogenesis step to provide a comprehensive understanding application of Fe-NPs. In addition, insight into the interactions between methanogens and Fe-NPs (e.g. role of methanogens, microbe interaction and gene transfer with Fe-NPs) beneficial for CH₄ production rate is provided. Microbial activity, inhibition effects and direct interspecies electron transfer through Fe-NPs have been extensively discussed. Finally, further studies towards detecting effective and optimised NPs based methods in the methanogenesis stage are reported.

Facile synthesis of recyclable 3D gelatin aerogel decorated with MIL-88B(Fe) for activation peroxydisulfate degradation of norfloxacin

The application of traditional powder catalysts is limited by particle agglomeration and difficult recovery. In this work, a three-dimensional porous aerogel catalyst for organic pollutants degradation in water by activating peroxydisulfate (PDS) was successfully synthesized, which was obtained via directly mixing of MIL-88B(Fe) with sol precursors followed by vacuum freeze-drying and low-temperature calcination. MIL-88B(Fe)/gelatin aerogel-150/PDS (MGA-150/PDS) system displayed satisfactory norfloxacin (NOR) degradation performance, which could remove 98.7% of NOR in 90 min. Its reaction rate constant was 23.2 times higher than the gelatin aerogel/PDS (GA/PDS) system. In addition, Electron paramagnetic resonance (EPR) results and radical trapping experiments revealed both radicals (SO₄^•-, •OH) and non-radical (¹O₂) pathways had participated in NOR degradation, of which •OH was dominant. Possible degradation pathways were proposed. Moreover, the high degradation efficiency of NOR by MGA-150 composites could still be reached more than 90.0% even after 10 cycles, and the morphology and chemical structure of MGA-150 composites exhibited no significant changes, indicating the arrestive stability of aerogel composites. This progress not only proposed an effective catalyst for PDS activation, but also expanded views for the design and development of 3D functional materials.

Long-term performance, membrane fouling behaviors and microbial community in a hollow fiber anaerobic membrane bioreactor (HF-AnMBR) treating synthetic terephthalic acid-containing wastewater

Purified terephthalic acid (PTA) wastewater with properties of poor biodegradation and high toxicity is produced from refining and synthesis of petrochemical products. In this study, a lab-scale hollow fiber membrane bioreactor (HF-AnMBR) fed with synthetic PTA wastewater was operated over 200 days with stepwise decreased hydraulic retention time (HRT) to investigate the long-term performance, membrane fouling mechanism and microbial community evolution. Results showed that a stable chemical oxygen demand (COD) removal rate of 65.8 ± 4.1% was achieved at organic loading rate of 3.1 ± 0.3 g-COD/L-reactor/d and HRT 24 h, under which the methane production rate reached 0.33 ± 0.02 L/L-reactor/d. Further shortening HRT, however, led to the decreased COD removal efficiency and low methane bioconversion. A mild membrane fouling occurred due to the production of colloidal biopolymers and the interaction between increased colloidal substances secreted/cracked by microorganisms and membrane interface. Further 16S rRNA analysis indicated that microbial diversity and richness had changed with the variation of HRT while Methanosaeta, and Methanolinea species were always the dominant methanogens responsible for methane production. The results verify that HF-AnMBR is an alternative technology for PTA wastewater treatment along with energy harvesting, and provide a new avenue toward sustainable petrochemical wastewater management.

Unraveling the role of polyferric chloride in anaerobic digestion of waste activated sludge

This study explored the effect of polyferric chloride (PFC) as a flocculant on waste activated sludge anaerobic digestion. The results verified that PFC has an inhibitory effect on methane production during anaerobic digestion. PFC with a concentration of 40 g/kg total suspended solids reduced methane production from 195 ± 2.10 to 156 ± 1.50 L/kg volatile suspended solids, a decrease of 20.0 ± 0.09%. PFC released hydroxyl polymers and Fe(III). Hydroxy polymers aggregated sludge flocs and hindered the release of dissolved organic matter. Fe(III) induced dissimilar iron reduction processes to contend with methyl-CoM for electrons, thereby further reducing methane production. In addition, PFC enriched iron-reducing bacteria and reduced the abundance of methanogens, resulting in microbial communities that are not conducive to methane production. This article puts forward innovative insights on the role of PFC in biological sludge treatment, which is expected to guide the flocculant selection during wastewater treatment.

High removal efficiencies of antibiotics and low accumulation of antibiotic resistant genes obtained in microbial fuel cell-constructed wetlands intensified by sponge iron

Using microbial fuel cells with constructed wetlands (MFC-CWs) for eliminating antibiotics has recently attracted extensive attention. However, antibiotic removal efficiencies in MFC-CWs must be enhanced, and the accumulation of antibiotic resistant genes (ARGs) remains an unmanageable issue. This study tries to enhance the antibiotic removal in synthetic wastewater and reduce ARGs by adding sponge iron (s-Fe⁰) and calcium peroxide to the anode and cathode of MFC-CWs, respectively, and/or simultaneously. The results demonstrated that adding s-Fe⁰ and calcium peroxide to MFC-CWs could improve the removal efficiencies of sulfamethoxazole (SMX) and tetracycline (TC) by 0.8-1.3% and 6.0-8.7%. Therein, s-Fe⁰ also significantly reduced 84.10-94.11% and 49.61-60.63% of total sul and tet genes, respectively. Furthermore, s-Fe⁰ improved the voltage output, power density, columbic efficiency, and reduced the internal resistance of reactors. The intensification to the electrode layers posed a significant effect on the microbial community composition and functions, which motivated the shift of antibiotic removal, accumulation of ARGs and bioelectricity generation in MFC-CWs. Given the overall performance of MFC-CWs, adding s-Fe⁰ to the anode region of MFC-CWs was found to be an effective strategy for removing antibiotics and reducing the accumulation of ARGs.

Nanoscale zero-valent iron inhibits the horizontal gene transfer of antibiotic resistance genes in chicken manure compost

Livestock manure has been identified as a significant hotspot for antibiotic resistance genes (ARGs). However, the impact of nanoscale zero-valent iron (nZVI) on the fate of ARGs during livestock manure composting remains poorly understood. Here, we investigated the evolution of ARGs in chicken manure compost exposed to 100 and 600 mg kg^-1 nZVI. The results showed that nZVI addition reduced the concentration of some antibiotics such as doxycycline and sulfamethoxazole. Furthermore, nZVI addition decreased the abundances of most ARGs at the end of composting, but nZVI dosage did not have any significant effect. The abundances of the dominant ARGs (sul1 and sul2) were significantly correlated to the class 1 integron-integrase gene (intI1). A network analysis revealed that the genera Bacteroides, Bacillus, Corynebacterium, Thiopseudomonas and Pseudomonas were the main potential hosts for multiple ARGs, and the decreased abundance of these bacteria contributed to the removal of ARGs. Structural equation models demonstrated that the reduction in intI1 played a predominant role in ARG removal. The nZVI also had direct effects on the intI1 abundance. These findings suggest that the addition of nZVI is a promising strategy to minimize ARG release in chicken manure compost.

Effect of Engineered Biomaterials and Magnetite on Wastewater Treatment: Biogas and Kinetic Evaluation

In this study, the principle of sustaining circular economy is presented as a way of recovering valuable resources from wastewater and utilizing its energy potential via anaerobic digestion (AD) of municipality wastewater. Biostimulation of the AD process was investigated to improve its treatability efficiency, biogas production, and kinetic stability. Addressing this together with agricultural waste such as eggshells (CE), banana peel (PB), and calcined banana peels (BI) were employed and compared to magnetite (Fe₃O₄) as biostimulation additives via 1 L biochemical methane potential tests. With a working volume of 0.8 L (charge with inoculum to substrate ratio of 3:5 v/v) and 1.5 g of the additives, each bioreactor was operated at a mesophilic temperature of 40 °C for 30 days while being compared to a control bioreactor. Scanning electron microscopy and energy dispersive X-ray (SEM/EDX) analysis was used to reveal the absorbent’s morphology at high magnification of 10 kx and surface pore size of 20.8 µm. The results showed over 70% biodegradation efficiency in removing the organic contaminants (chemical oxygen demand, color, and turbidity) as well as enhancing the biogas production. Among the setups, the bioreactor with Fe₃O₄ additives was found to be the most efficient, with an average daily biogas production of 40 mL/day and a cumulative yield of 1117 mL/day. The kinetic dynamics were evaluated with the cumulative biogas produced by each bioreactor via the first order modified Gompertz and Chen and Hashimoto kinetic models. The modified Gompertz model was found to be the most reliable, with good predictability.

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.

Mechanisms of metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic chloramphenicol wastewater treatment

Anaerobic wastewater treatment is a promising technology for refractory pollutant treatment. The nano zero-valent iron (nZVI) assisted anaerobic system could enhance contaminant removal. In this work, we added nZVI into an anaerobic system to investigate the effects on system performances and metabolic mechanism for chloramphenicol (CAP) wastewater treatment. As nZVI concentrations increased from 0 to 1 g/L, the CAP removal efficiency was appreciably improved from 46.5% to 99.2%, while the CH₄ production enhanced more than 20 times. The enhanced CAP removal resulted from the enrichments of dechlorination-related bacteria (Hyphomicrobium) and other functional bacteria (e.g., Zoogloea, Syntrophorhabdus) associated with refractory contaminants degradation. The improved CH₄ production was ascribed to the increases in fermentative-related bacteria (Smithella and Acetobacteroides), homoacetogen (Treponema), and methanogens. The increased abundances of anaerobic functional genes further verified the mechanism of CH₄ production. Furthermore, the abundances of potential hosts of antibiotic resistance genes (ARGs) were reduced under high nZVI concentration (1 g/L), contributing to ARGs attenuation. This study provides a comprehensive analysis of the mechanism in metabolic performance enhancement and ARGs attenuation during nZVI-assisted anaerobic CAP wastewater treatment.

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.

Effects of bentonite on antibiotic resistance genes in biogas slurry and residue from thermophilic and mesophilic anaerobic digestion of food waste

Anaerobic digestion (AD) processes of food waste (FW) have potential risk on environments due to the prevalence and dissemination of antibiotic resistance genes (ARGs). This study investigated the effect of bentonite on methane production and the abundance of ARGs in biogas slurry and residue during AD of FW. Results showed that methane production increased by 68.52% and 56.79% with 3 g/L and 5 g/L of bentonite in mesophilic and thermophilic digestion, respectively. Adding 5 g/L of bentonite effectively reduced the genes of ermB, ermF, tetQ, tetX, sul1, sul2 and intI1 with a range of 80.82% - 100.00% in biogas residue under mesophilic reactor. The abundance of ARGs and intI1 in biogas residue were lower than in slurry under both temperatures with 5 g/L of bentonite. Statistical analysis indicated that bentonite and temperature were main driver factors which could impact ARGs by influencing the abundance and structure of microbial communities.

Recent Approaches for the Production of High Value-Added Biofuels from Gelatinous Wastewater

Gelatin production is the most industry polluting process where huge amounts of raw organic materials and chemicals (HCl, NaOH, Ca2+) are utilized in the manufacturing accompanied by voluminous quantities of end-pipe effluent. The gelatinous wastewater (GWW) contains a large fraction of protein and lipids with biodegradability (BOD/COD ratio) exceeding 0.6. Thus, it represents a promising low-cost substrate for the generation of biofuels, i.e., H2 and CH4, by the anaerobic digestion process. This review comprehensively describes the anaerobic technologies employed for simultaneous treatment and energy recovery from GWW. The emphasis was afforded on factors affecting the biofuels productivity from anaerobic digestion of GWW, i.e., protein concentration, organic loading rate (OLR), hydraulic retention time (HRT), the substrate to inoculum (S0/X0) ratio, type of mixed culture anaerobes, carbohydrates concentration, volatile fatty acids (VFAs), ammonia and alkalinity/VFA ratio, and reactor configurations. Economic values and future perspectives that require more attention are also outlined to facilitate further advancement and achieve practicality in this domain.

Sustained effect of zero-valent iron nanoparticles under semi-continuous anaerobic digestion of sewage sludge: Evolution of nanoparticles and microbial community dynamics

The effects of adding zero-valent iron nanoparticles (nZVI) on the physicochemical, biological and biochemical responses of a semi-continuous anaerobic digestion of sewage sludge have been assessed. Two sets of consecutive experiments of 103 and 116 days, respectively, were carried out in triplicate. nZVI were magnetically retained in the reactors, and the effect of punctual doses (from 0.27 to 4.33 g L^-1) over time was studied. Among the different parameters monitored, only methane content in the biogas was significantly higher when nZVI was added. However, this effect was progressively lost after the addition, and in 5-7 days, the methane content returned to initial values. The increase in the oxidation state of nanoparticles seems to be related to the loss of effect over time. Higher dose (4.33 g L^-1) sustained positive effects for a longer time along with higher methane content, but this fact seems to be related to microbiome acclimation. Changes in microbial community structure could also play a role in the mechanisms involved in methane enhancement. In this sense, the microbial consortium analysis reported a shift in the balance among acetogenic eubacterial communities, and a marked increase in the relative abundance of members assigned to Methanothrix genus, recognized as acetoclastic species showing high affinity for acetate, which explain the rise in methane content in the biogas. This research demonstrates that biogas methane enrichment in semicontinuous anaerobic digesters can be achieved by using nZVI nanoparticles, thus increasing energy production or reducing costs of a later biogas upgrading process.

Role of anaerobic sludge digestion in handling antibiotic resistant bacteria and antibiotic resistance genes - A review

Currently, anaerobic sludge digestion (ASD) is considered not only for treating residual sewage sludge and energy recovery but also for the reduction of antibiotic resistance genes (ARGs). The current review highlights the reasons why antibiotic resistant bacteria (ARB) and ARGs exist in ASD and how ASD performs in the reduction of ARB and ARGs. ARGs and ARB have been detected in ASD with some reports indicating some of the ARGs can be completely removed during the ASD process, while other studies reported the enrichment of ARB and ARGs after ASD. This paper reviews the performance of ASD based on operational parameters as well as environmental chemistry. More studies are needed to improve the performance of ASD in reducing ARGs that are difficult to handle and also differentiate between extracellular (eARGs) and intracellular ARGs (iARGs) to achieve more accurate quantification of the ARGs.

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

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

Addition of nanoparticles increases the abundance of mobile genetic elements and changes microbial community in the sludge anaerobic digestion system

This study explored the fate of mobile genetic elements (MGEs) in anaerobic digestion (AD) system with four nanoparticles (NPs) added, including carbon NPs, Al₂O₃ NPs, ZnO NPs, and CuO NPs. 16S rRNA amplicon sequencing and quantitative PCR to investigate the microbial community, MGEs abundance and the potential host in the AD process. The results of high-throughput sequencing showed that ZnO NPs and CuO NPs significantly reduced the microbial diversity and significantly changed the microbial community structure. Simultaneously, the absolute abundance of MGEs increased by 145.01%, 159.67%, 354.70%, and 132.80% on the carbon NPs, Al₂O₃ NPs, ZnO NPs, and CuO NPs. The enrichment rate of tnpA-03 in ZnO NPs group was the highest, which could reach up to 2854.80%. Co-occurrence analysis revealed that Proteobacteria harbored the vast majority of MGEs followed by Firmicutes. Redundancy analysis and variation partitioning analysis showed that metabolites were the main factors that shifted the succession of bacterial communities. Moreover, there were significant positive correlations between metabolites and part MGEs (such as tnpA-01, tnpA-02, tnpA-03, tnpA-04, tnpA-05, tnpA-07 and ISCR1). This study provides a new perspective that NPs increase the risk of antibiotic resistance through MGEs during AD process.

Impacts of biochar on anaerobic digestion of swine manure: Methanogenesis and antibiotic resistance genes dissemination

This work aimed to study the effects of biochar on methanogenesis and antibiotic resistance genes (ARGs) fate during anaerobic digestion (AD) of swine manure (SM) was investigated. Under the optimal biochar dosage of 5-10%, methane yield was significantly improved by 25%, ascribing to the enhancement of direct interspecies electron transfer (DIET). Biochar addition alleviated the need of cytochrome-c as interspecies electron connection components and enriched the microbes involved in DIET. Defluviitoga, Thermovirga and Cloacibacillus were dominant with biochar addition and might participate in DIET together with Methanothrix. The abundance of parC, tetX, bla_CTX-M, bla_TEM, aac(6')-Ib-cr, ermB and tetW were significantly reduced in biochar treatments and intI1 removal increased by 15% in B-5% than the control. Structural equation models demonstrated that intI1 (λ = 0.59, P < 0.001) had the most standardized direct effects on ARGs, while biochar indirectly affected ARGs by changing intI1 (λ = 0.41, P < 0.01) and microbial structure (λ = -0.24, P < 0.05).

Intracellular versus extracellular antibiotic resistance genes in the environment: Prevalence, horizontal transfer, and mitigation strategies

Antibiotic resistance genes (ARGs) are present as both intracellular and extracellular fractions of DNA in the environment. Due to the poor yield of extracellular DNA in conventional extraction methods, previous studies have mainly focused on intracellular ARGs (iARGs). In this review, we evaluate the prevalence/persistence and horizontal transfer of iARGs and extracellular ARGs (eARGs) in different environments, and then explore advanced mitigation strategies in wastewater treatment plants (WWTPs) for preventing the spread of antibiotic resistance in the environment. Although iARGs are the main fraction of ARGs in nutrient-rich environments, eARGs are predominant in receiving aquatic environments. In such environments, natural transformation of eARGs occurs with a comparable frequency to conjugation of iARGs. Further, eARGs can be adsorbed by soil and sediments particles, protected from DNase degradation, and consequently persist longer than iARGs. Collectively, these characteristics emphasize the crucial role of eARGs in the spread of antibiotic resistance in the environment. Fate of iARGs and eARGs through advanced treatment technologies (disinfection and membrane filtration) indicates that different mitigation strategies may be required for each ARG fraction to be significantly removed. Finally, comprehensive risk assessment is needed to evaluate/compare the effect of iARGs versus eARGs in the environment.