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

Effects of nitrogen fertilization on antibiotic resistance gene spread from soil to floodwater in paddy fields

The environmental risks associated with antibiotic resistance genes (ARGs) have drawn increasing attention. Here, we investigated the spread of ARGs from soil to floodwater in paddy fields by conducting 5-year field experiments having four treatments: control, reduced N fertilization, conventional N fertilization (CN) and plant-based organic N fertilization (ON). In comparison to the control treatment, CN and ON treatments significantly increased the total N content by 26.8%-36.6% in soil and 42.7%-67.4% in floodwater, respectively. Additionally, ON treatments increased the concentrations of free-floating and particle-attached mobile gene elements in floodwater by 78.6% and 32.7%, respectively. Redundancy and linear regression analyses indicated that total N and NH4+ in soil and mobile gene elements in floodwater were closely connected with the levels of ARGs in floodwater. Correspondingly, CN and ON treatments increased total abundances of ARGs in floodwater to 1.26 and 1.46 times, respectively. The network analysis further revealed that ARGs, such as MexW, tetAB and optrA, could be carried by the hosts of ARGs (such as Chloroflexi, Gemmatimonadetes and Nitrospirae) and spread from soil to particle-attached floodwater fractions through suspended solids. Moreover, structure equation models indicated that the evolution of ARGs in free-floating fractions could dominate the propagation of ARGs in floodwater. These findings provide valuable insights into the propagation of ARGs in paddy fields and highlight nutrient management in paddy field floodwater as a potentially effective strategy to mitigate the spread of ARGs.

Biogas production enhancement from anaerobic digestion with magnetic biochar: Insights into the functional microbes and DIET

The application of magnetic biochar in anaerobic digestion (AD) has gained increasing attention. However, the underlying mechanisms remain insufficiently understood. This study systematically investigated the effects of magnetic biochar on functional microbial communities involved in methanogenesis and elucidated its role in promoting direct interspecies electron transfer (DIET) within AD systems. The addition of 40 mg g-1 TSadded of magnetic biochar significantly enhanced methane production by 42.21 %, reaching 223.08 mL g-1 TS with highest organic matter degradation efficiency. Microbial community analysis showed that magnetic biochar significantly enriched microorganisms associated with hydrolysis, acidogenesis, and methanogenesis, as well as electroactive microorganisms' abundance such as Geobacter spp., Syntrophus spp., P. aestuarii, and M. harundinacea, providing direct evidence for the DIET process of AD with magnetic biochar. Furthermore, the abundance of key genes involved in the DIET, including pilA, Fpo, and the genes encoded outer-membrane c-type cytochromes, was respectively upregulated by 44.49 %, 22.04 %, and 37.6 % in the presence of magnetic biochar. These findings suggest that magnetic biochar enhances the production of conductive pili and cytochrome c, facilitating extracellular electron transfer between syntrophic microorganisms. This accelerated electron transfer promotes CO2 reduction to CH4, ultimately improving methane production efficiency in the AD system. Moreover, the enhancement of hydrogenotrophic methanogenesis was particularly pronounced with magnetic biochar, further contributing to the improved AD performance. This study provides novel mechanistic insights into biochar-mediated DIET, offering a theoretical basis for optimizing biochar applications in AD.

Deciphering the synergistic effects and mechanisms of biochar and magnetite contained in magnetic biochar for enhancing methane production in anaerobic digestion of waste activated sludge

Adding conductive materials is one of the most extensive enhancement strategies while treating waste activated sludge via anaerobic digestion. Magnetic biochar (MBC), as one composite conductive material, is capable of enhancing methane yield and production rate because of its favorable characteristics. However, whether the synergistic effects formed or not between biochar and magnetite contained in MBC on anaerobic digestion is still unclear. This study investigated the synergistic effects and corresponded mechanisms of biochar and magnetite contained in MBC with semi-continuous anaerobic digestion mode. Results showed that the co-addition of biochar and magnetite performed non-synergistic effects on methane production potential, with decrease ratios of 2.2 % and 7.4 % respectively compared to that in biochar and magnetite groups. Interestingly, the biochar and magnetite contained in MBC formed synergistic effects, with an extra improvement of 5.5 % compared to the sum of those obtained in biochar and magnetite groups. The synergistic effects came from efficient hydrolysis and acidogenesis stages, including the thorough degradation of soluble organic matters and the rapid conversion of acetic acids. MBC also produced synergistic effects on the hydrophilia and redox properties of extracellular polymeric substances, the activities of enzymes involved in interspecies electron transfer, and the contents of adenosine triphosphate (ATP). Specifically, the enhancement potential contributed by MBC exceeded the total enhancement potential contributed by biochar and magnetite, with the extra enhancement ratios of 13.1 % and 19.4 % for cytochrome c and ATP, thus, the biochar and magnetite contained in MBC formed synergistic effects for promoting electron transmembrane and transfer from kinetic aspects. The correlation coefficient between methane production performance and the microbial electron transfer activity reached 0.96, correspondingly, the highest electron transfer activity of microorganisms was presented in MBC. As for microbial communities, the functional and electro-active microorganisms were enriched with the addition of MBC, such as Peptoclostridium, Anaerolineaceae, Methanosarcina, and Methanosaeta, which facilitated the conversion of organic matters and established direct interspecies electron transfer methanogenesis. The findings of this study revealed the synergistic effects and mechanisms of biochar and magnetite contained in magnetic biochar in enhancing sludge anaerobic digestion, and provided an effective strategy to recover bioenergy from waste activated sludge, potentially boosting carbon neutrality in wastewater treatment.

Enhanced anaerobic digestion of waste activated sludge using magnetite-modified sludge ceramsite: Performance and microbial dynamics

The effect of magnetite-modified sludge ceramsite on anaerobic digestion of sludge was systematically investigated in this study. The results revealed that the addition of magnetite significantly altered the properties of the ceramsite, while magnetite-modified ceramsite significantly altered the sludge anaerobic digestion process. Biochemical methane production potential experiments demonstrated that the cumulative methane production of the experimental group with moderate ceramsite addition was enhanced by 17.8% compared to the control group without ceramsite. This enhancement was attributed primarily to the favorable pore structure and biocompatibility of the modified ceramsite. The incorporation of magnetite facilitated the enrichment of microorganisms and the reduction of Fe(III), thereby promoting anaerobic digestion. Moreover, the ceramsite exhibited strong buffering capacity, contributing to the enhanced stability of the digestive system. Microbiological analyses revealed that the addition of ceramsite significantly altered the microbial community. Appropriate ceramsite addition resulted in the enrichment of bacteria associated with organic matter degradation and methanogenesis. Furthermore, potential iron-reducing bacteria (Clostridium_sensu_stricto) and bacteria capable of direct interspecies electron transfer (Syntrophomonas) were also enriched in the anaerobic system. This study demonstrates a viable approach for the efficient resource utilization of sludge.

The key role of magnetic iron-to-biochar mass ratios in the dissipation of oxytetracycline and its resistance genes in soils with and without biodegradable microplastics

There are challenges involved in the synergistic dissipation of antibiotics and antibiotic resistance genes (ARGs) in soil because ARGs are affected by not only the selective pressure of antibiotics but also microbial community succession and co-existing pollutants. Here, magnetic biochars (MBCs) at various magnetic iron-to-biochar mass ratios (3:1, 2:1, 1:1, 1:2, 1:3, 1:5, and 1:7) were synthesized to develop a strategy for the synergistic dissipation of oxytetracycline (OTC) and its resistance gene (tet) in soils with and without polybutylene adipate terephthalate (PBAT) microplastics (MPs). The results showed that MBC12 (1:2) achieved the greatest dissipation efficiencies of OTC in soils without and with PBAT MPs (95.27% and 94.50%, respectively). The reductive degradation of OTC via promoting the electron transfer during conversion between Fe(III) and Fe(II) overwhelmed biodegradation of OTC. MBCs effectively hindered the spread of tet in soil without PBAT MPs, with the efficiencies more than 60%; but they had little influence on its spread in soil with PBAT MPs, excluding MBC15 (1:5). The absolute abundance of tet, regardless of PBAT MPs, just significantly positively correlated with Serratia (the added exogenous tet-host bacterium), indicating that MBCs inhibited the horizontal transfer of tet at the inter-genus level. Down-regulating the degradation/utilization/assimilation metabolic function by MBCs (excluding MBC31, 3:1) contributed to the hindering class 1 integron gene (intI1)-driven tet propagation. After considering efficiency, cost and toxic effects, MBC12 (1:2) was recommended to use for synergistic dissipation of OTC and tet in soils without and with PBAT MPs.

Impact of earthworms on suppressing dissemination of antibiotic resistance genes during vermicomposting treatment of excess sludge

Earthworms play a crucial role in suppressing the dissemination of antibiotic resistance genes (ARGs) during vermicomposting. However, there is still a lack of how earthworms influence the spread of ARGs. To address this gap, a microcosm experiment was conducted, incorporating earthworms and utilizing metagenomics and quantitative PCR to assess the impact of earthworms on microbial interactions and the removal of plasmid-induced ARGs. The findings revealed that vermicomposting led to a reduction in the relative abundance of ARGs by altering microbial communities and interactions. Significantly, vermicomposting demonstrated an impressive capability, reducing 92% of ARGs donor bacteria and impeding the transmission of 94% of the RP4 plasmid. Furthermore, through structural equation model analysis, it was determined that mobile genetic elements and environmental variables were the primary influencers of ARG reduction. Overall, this study offers a fresh perspective on the effects of vermicomposting and its potential to mitigate the spread of ARGs.

Green synthesis of Fe3O4@ceramsite from sludge improving anaerobic digestion performance of waste activated sludge

Anaerobic digestion (AD) is a promising technique for waste management, which can achieve sludge stabilization and energy recovery. This study successfully prepared Fe3O4@ceramsite from WAS and applied it as an additive in sludge digestion, aiming to improve the conversion of organics to biomethane efficiency. Results showed that after adding the Fe3O4@ceramsite, the methane production was enhanced by 34.7% compared with the control group (88.0 ± 0.1 mL/g VS). Further mechanisms investigation revealed that Fe3O4@ceramsite enhanced digesta stability by strong buffering capacity, improved sludge conductivity, and promoted Fe (III) reduction. Moreover, Fe3O4@ceramsite has a larger surface area and better porous structure, which also facilitated AD performance. Microbial community analysis showed that some functional anaerobes related to AD such as Spirochaeta and Smithella were enriched with Fe3O4@ceramsite treatment. Potential syntrophic metabolisms between syntrophic bacteria (Syntrophomonas, associated with DIET) and methanogens were also detected in the Fe3O4@ceramsite treatment AD system.

Co-occurrence of dominant bacteria and methanogenic archaea and their metabolic traits in a thermophilic anaerobic digester

Thermophilic anaerobic digestion (TAD) represents a promising biotechnology for both methane energy production and waste stream treatment. However, numerous critical microorganisms and their metabolic characteristics involved in this process remain unidentified due to the limitations of culturable isolates. This study investigated the phylogenetic composition and potential metabolic traits of bacteria and methanogenic archaea in a TAD system using culture-independent metagenomics. Predominant microorganisms identified in the stable phase of TAD included hydrogenotrophic methanogens (Methanothermobacter and Methanosarcina) and hydrogen-producing bacteria (Coprothermobacter, Acetomicrobium, and Defluviitoga). Nine major metagenome-assembled genomes (MAGs) associated with the dominant genera were selected to infer their metabolic potentials. Genes related to thermal resistance were widely found in all nine major MAGs, such as the molecular chaperone genes, Clp protease gene, and RNA polymerase genes, which may contribute to their predominance under thermophilic condition. Thermophilic temperatures may increase the hydrogen partial pressure of Coprothermobacter, Acetomicrobium, and Defluviitoga, subsequently altering the primary methanogenesis pathway from acetoclastic pathway to hydrogenotrophic pathway in the TAD. Consequently, genes encoding the hydrogenotrophic methanogenesis pathway were the most abundant in the recovered archaeal MAGs. The potential interaction between hydrogen-producing bacteria and hydrogenotrophic methanogens may play critical roles in TAD processes.

Metagenomic insights into the toxicity of carbamazepine to functional microorganisms in sludge anaerobic digestion

Contaminants of emerging concern (CECs) contained in sludge, such as carbamazepine, may be toxic to microorganisms and affect the biogenesis of methane during anaerobic digestion. In this study, different scales of anaerobic digesters were constructed to investigate the inhibitory effect of carbamazepine. Results showed that carbamazepine reduced methane production by 11.3 % and 62.1 % at concentrations of 0.4 and 2 mg/g TS, respectively. Carbamazepine hindered the dissolution of organic matter and the degradation of protein. Carbamazepine inhibited some fermentative bacteria, especially uncultured Aminicenantales, whose abundance decreased by 9.5-93.4 % under carbamazepine stress. It is worth noting that most prior studies investigated the effects of CECs only based on well-known microorganisms, ignoring the metabolisms of uncultured microorganisms. Genome-predicted metabolic potential suggested that 54 uncultured metagenome-assembled genomes (MAGs) associated with acidogenesis or acetogenesis. Therein, uncultured Aminicenantales related MAGs were proved to be acetogenic fermenters, their significant reduction may be an important reason for the decrease of methane production under carbamazepine stress. The toxicity of carbamazepine to microorganisms was mainly related to the overproduction of reactive oxygen species. This study elucidates the inhibition mechanism of carbamazepine and emphasizes the indispensable role of uncultured microorganisms in anaerobic digestion.

Peracetic acid pretreatment improves biogas production from anaerobic digestion of sewage sludge by promoting organic matter release, conversion and affecting microbial community

Peracetic acid (PAA) pretreatment is considered as a novel and effective chemical pretreatment method for sludge. However, there is little information available on potential mechanisms of how PAA pretreatment affects sludge anaerobic digestion (AD). To fill the knowledge gap, this study investigated the effects and potential mechanisms of PAA pretreatment on sludge AD systems from physicochemical and microbiological perspectives. Batch experiments resulted that biogas production was enhanced by PAA pretreatment and the highest cumulative biogas yield (297.94 mL/g VS (volatile solid)) was obtained with 2 mM/g VS of PAA pretreatment. Kinetic model analysis illustrated that the PAA pretreatment improved the biogas potential (Pt) of sludge AD, but prolonged the lag phase (λ) of AD. Mechanistic studies revealed that reactive oxygen species (ROS) (HO•, O2-•, 1O2 and CH3C(O)OO•) were the major intermediate products of PAA decomposition. These ROS effectively promoted the decomposition and solubilization of sludge, and provided more biodegradable organic matter for the following AD reactions. 16S rRNA amplicon sequencing showed that some functional microorganisms associated with hydrolysis, acidogenesis, acetogenesis as well as methanogenesis, such as Hydrogenispora, Romboutsia, Longivirga, Methanosarcina and Methanosaet, were significantly enriched in reactors pretreated with PAA. Redundancy analysis and variation partitioning analysis indicated that functional microorganisms were significantly correlated with intermediate metabolites (soluble carbohydrate, soluble protein, soluble chemical oxygen demand and volatile fatty acids) and cumulative biogas production. This study provides a fresh understanding of the effects and mechanisms of PAA pretreatment on sludge AD, updates the insights into the response of functional microorganisms to PAA pretreatment, and the findings obtained might provide a fundamental basis for chemical pretreatment of sludge AD using oxidants.

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.

Superparamagnetic Iron Oxide Nanoparticles as Additives for Microwave-Based Sludge Prehydrolysis: A Perspective

Wastewater treatment plants are critical for environmental pollution control. The role that they play in protecting the environment and public health is unquestionable; however, they produce massive quantities of excess sludge as a byproduct. One pragmatic approach to utilizing excess sludge is generating methane via anaerobic digestion. For this, a prehydrolysis step can significantly improve digestion by increasing biogas quality and quantity while decreasing final sludge volumes. One of the many prehydrolysis approaches is to deliver heat into sludge via microwave irradiation. Microwave-absorbing additives can be used to further enhance thermal degradation processes. However, the implications of such an approach include potential release of said additive materials into the environment via digested sludge. In this perspective, we present and discuss the potential of superparamagnetic iron oxide nanoparticles (SPIONs) as recoverable, hyperreactive microwave absorbers for sludge prehydrolysis. Due to their size and characteristics, SPIONs pack spin electrons within a single domain that can respond to the magnetic field without remanence magnetism. SPIONs have properties of both paramagnetic and ferromagnetic materials with little to no magnetic hysteresis, which can enable their rapid recovery from slurries, even in complicated reactor installations. Further, SPIONs are excellent microwave absorbers, which result in high local heat gradients. This perspective introduces the vision that SPION properties can be tuned for desirable dielectric heating and magnetic responses while maintaining material integrity to accomplish repeated use for microwave-enhanced pretreatment.