Effects of graphite, graphene, and graphene oxide on the anaerobic co-digestion of sewage sludge and food waste: Attention to methane production and the fate of antibiotic resistance genes

Unlocking anaerobic digestion potential via extracellular electron transfer by exogenous materials: Current status and perspectives

The efficiency of energy transfer among microorganisms presents a substantial hurdle for the widespread implementation of anaerobic digestion techniques. Nonetheless, recent studies have demonstrated that enhancing the extracellular electron transfer (EET) can markedly enhance this efficiency. This review highlights recent advancements in EET for anaerobic digestion and examines the contribution of external additives to fostering enhanced efficiency within this context. Diverse mechanisms through which additives are employed to improve EET in anaerobic environments are delineated. Furthermore, specific strategies for effectively regulating EET are proposed, aiming to augment methane production from anaerobic digestion. This review thus offers a perspective on future research directions aimed at optimizing waste resources, enhancing methane production efficiency, and improving process predictability in anaerobic digestion.

The ambivalent role of graphene oxide in anaerobic digestion: A review

The capability of graphene oxide (GO) to enhance direct interspecies electron transfer (DIET) and improve anaerobic digestion (AD) performance is gaining attention in AD literature. The present review discusses the implications of GO and its ambivalent role in AD. Under anaerobic conditions, GO is rapidly converted to biologically reduced graphene oxide (bioRGO) through microbial respiration. GO addition could promote the release of extracellular polymeric substances and lead to toxic effects on anaerobic microorganisms. However, further research is needed to determine the GO toxic concentration thresholds. GO application can impact biogas production and organic micropollutants removal of anaerobic digesters. Nevertheless, most of the studies have been conducted at batch scale and further work in continuously operated anaerobic digesters is still needed. Finally, the review evaluates the economic potential of GO application in AD systems. Overall, this review lays the foundations to improve the applicability of GO in future full-scale digesters.

Enhancing methane production from corn straw via illumination-assisted Fe3O4/g-C3N4 nanocomposite in anaerobic digestion

This study proposes a novel anaerobic digestion (AD) strategy combining recyclable photoactivated nanomaterials with illumination to enhance electronic transfer for anaerobic microorganisms. Results showed that 7000 Lux illumination increased methane production yield and rate. Incorporating Fe3O4 into graphite carbon nitride (g-C3N4) created a recyclable Fe3O4/g-C3N4 (FG) nanocomposite with improved light absorption, conductivity, redox properties, and methane promotion. The highest methane yield from corn straw was achieved with 7000 Lux and 1.5 g/L FG nanocomposite, 22.6% higher than the dark control. The AD system exhibited increased adenosine triphosphate content, improved redox performance, reduced electron transfer resistance, and higher photocurrent intensity. These improvements bolstered the microorganisms and key genes involved in hydrolysis and acidification, which in turn optimized the acetoclastic pathway. Furthermore, this strategy promoted microorganisms associated with direct interspecies electron transfer, fostering a favorable environment for methanogenic activities, paving the way for future anaerobic reactor developments.

Chlorinated organophosphorus flame retardants induce the propagation of antibiotic resistance genes in sludge fermentation systems: Insight of chromosomal mutation and microbial traits

Waste activated sludge (WAS) is a critical reservoir for antibiotic resistance genes (ARGs) due to the prevalent misuse of antibiotics. Horizontal gene transfer (HGT) is the primary mechanism for ARGs spread through mobile genetic elements (MGEs). However, the role of non-antibiotic organophosphorus flame retardants (Cl-OFRs) in ARG transmission in the WAS fermentation system remains unclear. This study examines the effects of tris(2-chloroethyl) phosphate (TCEP), a representative Cl-OFR, on ARG dynamics in WAS fermentation using molecular docking and metagenomic analysis. The results showed a 33.4 % increase in ARG abundance in the presence of TCEP. Interestingly, HGT did not appear to be the primary mechanism of ARG dissemination under TCEP stress, as evidenced by a 2.51 % decrease in MGE abundance. TCEP binds to sludge through hydrogen bonds with a binding energy of - 3.6 kJ/mol, leading to microbial damage and an increase in the proportion of non-viable cells. This interaction prompts a microbial shift toward Firmicutes with thick cell walls, which are significant ARG carriers. Additionally, TCEP induces chromosomal mutations through oxidative stress and the SOS response, contributing to ARG formation. Microorganisms also develop multidrug resistance mechanisms to expel TCEP and mitigate its toxicity. This study provides a comprehensive understanding of Cl-OFRs effects on the ARGs fates in WAS fermentation system and offers guidance for the safe and efficient treatment of Cl-OFRs and WAS.

Metagenomic analysis reveals the mechanisms of biochar supported nano zero-valent iron in two-phase anaerobic digestion of food waste: microbial community, CAZmey, functional genes and antibiotic resistance genes

The mechanisms of biochar supported nano zero-valent iron (BC/nZVI) on two-phase anaerobic digestion of food waste were investigated. Results indicated that the performance of both acidogenic phase and methanogenic phase was effectively facilitated. BC/nZVI with the amount of 120 mg/L increased methane production by 32.21%. In addition, BC/nZVI facilitated direct interspecies electron transfer (DIET) between Geobacter and methanogens. Further analysis showed that BC/nZVI increased the abundance of most CAZymes in acidogenic phase. The study also found that BC/nZVI had positive effects on metabolic pathways and related functional genes. The abundances of acdA and ackA in acidogenic phase were increased by 151.75% and 36.26%, respectively, and the abundances of pilA and TorZ associated with DIET were also increased. Furthermore, BC/nZVI mainly removed IMP-12, CAU-1, cmeB, ErmR, MexW, ErmG, Bla2, vgaD, MuxA, and cpxA from this system, and reduced the antibiotic resistance genes for antibiotic inactivation resistance mechanisms.

Effects of carbon-based conductive materials on semi-continuous anaerobic co-digestion of organic fraction of municipal solid waste and waste activated sludge

Organic fraction of municipal solid waste (OFMSW) and waste activated sludge (WAS) are the most produced organic waste streams in urban centres. Their anaerobic co-digestion (AcoD) allows to generate methane (CH4) and digestate employable as renewable energy source and soil amendment, respectively, fully in accordance with circular bioeconomy principles. However, the widespread adoption of such technology is limited by relatively low CH4 yields that fail to bridge the gap between benefits and costs. Among strategies to boost AcoD of OFMSW and WAS, use of conductive materials (CMs) to promote interspecies electron transfer has gained increasing attention. This paper presents one of the few experimental attempts of investigating the effects of four different carbon(C)-based CMs (i.e., granular activated carbon - GAC, graphite - GR, graphene oxide - GO, and carbon nanotubes - CNTs) separately added in semi-continuous AcoD of OFMSW and thickened WAS. The presence of C-based CMs has been observed to improve CH4 yield of the control process. Specifically, after 63 days of operation (concentrations of GAC and GR of 10.0 g/L and of GO and CNTs of 0.2 g/L), 0.186 NL/gVS, 0.191 NL/gVS, 0.203 NL/gVS, and 0.195 NL/gVS of CH4 were produced in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.177 NL/gVS produced in the control process. Likewise, at the end of the test (i.e., after 105 days at concentrations of C-based CMs half of the initial ones), CH4 yields were 0.193 NL/gVS, 0.201 NL/gVS, 0.211 NL/gVS, and 0.206 NL/gVS in reactors supplemented with GAC, GR, GO, and CNTs, respectively, compared to 0.186 NL/gVS of the control process. Especially with regard to GR, GO, and CNTs, results obtained in the present study represent a significant advance of the knowledge on the effects of such C-based CMs to realistic and scalable AD process conditions respect to previous literature.

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.

Feasibility of anaerobic co-digestion of biodegradable plastics with food waste, investigation of microbial diversity and digestate phytotoxicity

The effects of biodegradable plastics of different thicknesses (30 and 40 μm) and sizes (20 × 20, 2 × 2, and 1 × 1 mm) on anaerobic digestion of food waste and digestate phytotoxicity were investigated. Methane productions (38 days) for the groups with 20 × 20, 2 × 2, and 1 × 1 mm of 30 μm plastics were 92.46, 138.27, and 259.95 mL/gVSremoval, respectively which are nearly 58 % higher than the control group (58.86 mL/gVSremoval). Methane production in 40 μm plastics groups was lower than in 30 μm groups of equal size. All sizes of 30 µm plastics promoted substrate hydrolysis, acidification, and relative abundance of key hydrolytic bacteria and methanogens. Phytotoxicity tests results showed that seed root elongation was inhibited in groups with 40 μm plastics. In conclusion, 30 μm biodegradable plastics were more suitable for anaerobic digestion with food waste than 40 μm.

Petroleum coke supplementation for enhanced biogas production and phosphate removal under mesophilic conditions

The use of carbon-based conductive materials has been shown to lead to an increase in biogas and methane yields during anaerobic digestion (AD). The effect of these additives on AD using synthetic substrates has been extensively studied, yet their significance for wastewater sludge digestion has not been adequately investigated. Therefore, the aim of this research was to optimize the concentration of petroleum coke (PC) that is a waste by-product of oil refineries, for the anaerobic digestion of wastewater sludge and investigation of phosphate removal in the AD process in the mesophilic temperature range. According to the results of the experiments, supplementing reactors with PC could significantly improve biogas and methane production. Supplementation of reactors with 1.5 g/L PC led to 23.40 ± 0.26% and 42.55 ± 3.97% increase in biogas production and methane generation, respectively. Moreover, the average volatile solids (VS), phosphate, and chemical oxygen demand (COD) removals were 43.43 ± 0.73, 46.74 ± 0.77%, and 60.40 ± 0.38%, respectively.

Stimulating anaerobic digestion to degrade recalcitrant organic pollutants: Potential role of conductive materials-led direct interspecies electron transfer

This review aims to provide a comprehensive understanding of the potential of CMs-dominated DIET in the degradation of recalcitrant organic pollutants in AD. The review covers the mechanisms and efficiencies of recalcitrant organic pollutant degradation by CMs-dominated DIET, the comparison of degradation pathways between DIET and chemical treatment, recent insights on DIET-enhanced degradation, and the evaluation of the potential and future development of CMs-dominated DIET. The review emphasizes the importance of coupled syntrophic microorganisms, electron flux, and physicochemical properties of CMs in enhancing the degradation performance of AD. Additionally, it highlights the advantages of DIET-led syntrophic metabolism over traditional oxidation technologies in terms of environmental friendliness and efficiency. Finally, the review acknowledges the potential risks associated with introducing CMs into AD systems and provides guidance for waste treatment and energy recovery.

Conductive materials enhance microbial salt-tolerance in anaerobic digestion of food waste: Microbial response and metagenomics analysis

Previous studies have shown that high salinity environments can inhibit anaerobic digestion (AD) of food waste (FW). Finding ways to alleviate salt inhibition is important for the disposal of the growing amount of FW. We selected three common conductive materials (powdered activated carbon, magnetite, and graphite) to understand their performance and individual mechanisms that relieve salinity inhibition. Digester performances and related enzyme parameters were compared. Our data revealed that under normal and low salinity stress conditions, the anaerobic digester ran steady without significant inhibitions. Further, the presence of conductive materials promoted conversion rate of methanogenesis. This promotion effect was highest from magnetite > powdered activated carbon (PAC) > graphite. At 1.5% salinity, PAC and magnetite are beneficial in maintaining high methane production efficiency while control and the graphite added digester acidified and failed rapidly. Additionally, metagenomics and binning were used to analyze the metabolic capacity of the microorganisms. Some species enriched by PAC and magnetite possessed higher cation transport capacities and were to accumulate compatible solutes. PAC and magnetite promoted direct interspecies electron transference (DIET) and syntrophic oxidation of butyrate and propionate. Also, the microorganisms had more energy available to cope with salt inhibition in the PAC and magnetite added digesters. Our data imply that the promotion of Na+/H+ antiporter, K+ uptake, and osmoprotectant synthesis or transport by conductive materials may be crucial for their proliferation in highly stressful environments. These findings will help to understand the mechanisms of alleviate salt inhibition by conductive materials and help to recover methane from high-salinity FW.

Characteristics of digested sludge-derived biochar for promoting methane production during anaerobic digestion of waste activated sludge

This study investigated a novel method for enhancing methane production during anaerobic digestion of waste activated sludge with digested sludge-derived biochar (DSBC). Using response surface methodology, the following process conditions for DSBC synthesis were optimized: heating rate = 13.23 °C/min, pyrolysis temperature = 516 °C, and heating time = 192 min. DSBC significantly enhanced the methane production by 48 % and improved key coenzyme activity that accelerated the bioconversion of organic matter while promoting the decomposition and transformation of volatile fatty acids. Consequently, the lag period of methane production was shortened to 4.89 days, while the average proportion of methane greatly increased to 73.22%. Thus, DSBC could facilitate efficient methanogenesis in the anaerobic system by promoting electron transfer between syntrophic partners through the charge-discharge cycle of surface oxygen-containing functional groups. The study provides a reference for the resource utilization of anaerobic sludge residues and efficient anaerobic methanogenesis from sludge.

Untargeted metabolomics elucidated biosynthesis of polyhydroxyalkanoate by mixed microbial cultures from waste activated sludge under different pH values

Waste activated sludge has been frequently used as mixed substrate to produce polyhydroxyalkanoate (PHA). However, insufficient research on microbial metabolism has led to difficulties in regulating PHA accumulation in mixed microbial cultures (MMCs). To explore the variation of functional genes during domestication and the effect of different pH conditions on metabolic pathways during PHA accumulation, MMCs were domesticated by adding acetate and propionate with aerobic dynamic feeding strategy for 60 days. As the domestication progressed, the microbial community diversity declined and PHA-producing bacteria, Brevundimonas, Dechloromonas and Hyphomonas, were enriched. Through bacterial function prediction by PICRUSt the gene rpoE involved in starvation resistance of bacteria was enriched after the domestication. The pH value of 8.5 was the best condition for PHA accumulation in MMCs, under which a maximum PHA content reached 23.50% and hydroxybutyric (HB)/hydroxyvaleric (HV) reached 2.22. Untargeted metabolomics analysis exhibited that pH conditions of 7 and 8.5 could promote the up-regulation of significant differential metabolites, while higher alkaline conditions caused the inhibition of metabolic activity. Functional annotation showed that pH condition of 8.5 significantly affected Pyrimidine metabolism, resulting in an increase in PHA production. Regarding the pathways of PHA biosynthesis, acetoacetate was found to be significant in the metabolism of hydroxybutyric, and the alkaline condition could restrain the conversion from hydroxybutyric (HB) to the acetoacetate to protect PHB accumulation in MMCs compared with neutral condition. Taken together, the present results can advance the fundamental understanding of metabolic function in PHA accumulation under different pH conditions.

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.

Effects of concentration-dependent graphene on maize seedling development and soil nutrients

The long-term use of chemical fertilizers to maintain agricultural production has had various harmful effects on farmland and has greatly impacted agriculture's sustainable expansion. Graphene, a unique and effective nanomaterial, is used in plant-soil applications to improve plant nutrient uptake, reduce chemical fertilizer pollution by relieving inadequate soil nutrient conditions and enhance soil absorption of nutrient components. We investigated the effects of graphene amendment on nutrient content, maize growth, and soil physicochemical parameters. In each treatment, 5 graphene concentration gradients (0, 25, 50, 100, and 150 g kg^-1) were applied in 2 different types (single-layer and few-layers, SL and FL). Soil aggregates, soil accessible nutrients, soil enzyme activity, plant nutrients, plant height, stem diameter, dry weight, and fresh weight were all measured throughout the maize growth to the V3 stage. Compared to the control (0 g kg^-1), we found that graphene increased the percentage of large agglomerates (0.25-10 mm) in the soil and significantly increased the geometric mean diameter (GMD) and mean weight diameter (MWD) values of > 0.25 mm water-stable agglomerates as the increase of concentration. Soil available nutrient content (AN, AP, and AK) increased, peaking at 150 g kg^-1. Graphene boosted nutrient absorption by maize plants, and aboveground total nitrogen (TN), total phosphorus (TP), and total potassium (TK) contents rose with the increasing application, which raised aboveground fresh weight, dry weight, plant height, and stalk thickness. The findings above confirmed our prediction that adding graphene to the soil may improve maize plant biomass by enhancing soil fertility and improving the soil environment. Given the higher manufacturing cost of single-layer graphene and the greater effect of few-layer graphene on soil and maize plants at the same concentration, single-layer graphene and few-layer graphene at a concentration of 50 g kg^-1 were the optimal application rates.

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%.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Metagenomics insight into bioaugmentation mechanism of Propionibacterium acidipropionici during anaerobic acidification of kitchen waste

In the present study, a biochemical strategy for improving propionic acid production from kitchen waste acidification by bioaugmentation with Propionibacterium acidipropionici (P. acidipropionici) was investigated. When the inoculum of P. acidipropionici was 30% (w/w) of the seeding sludge, the propionic acid production increased by 79.57%. Further, bioaugmentation improved the relative abundance of Firmicute and Actinobacteria. The results of metagenomic analysis further reveal that the ATP-binding cassette (ABC) transporters and all related pathways of Propanoate metabolism (ko00640) were enriched when P. acidipropionici was added. For Propanoate metabolism, most functional genes involved in the conversion from Glycolysis / Gluconeogenesis (ko00010) to Propanoyl-CoA and conversion from Propanoyl-CoA to propionic acid were enhanced after bioaugmentation with P. acidipropionici, thereby promoting propionic acid production. As such, bioaugmentation with P. acidipropionici was effective in the anaerobic acidification of kitchen waste for propionic acid production.

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.

Recent advances in conductive materials amended anaerobic co-digestion of food waste and municipal organic solid waste: Roles, mechanisms, and potential application

Recently, conductive materials (i.e., carbon-based and iron-based materials) as a feasible and attractive approach have been introduced to anaerobic co-digestion (ACoD) system for promoting its performance and stability through direct interspecies electron transfer. Owing to the key roles of conductive materials in ACoD process, it is imperative to gain a profound understanding of their specific functions and mechanisms. Here, this review critically examined the state of the art of conductive materials assisted ACoD of food waste and common municipal organic solid waste. Then, the fundamental roles of conductive materials on ACoD enhancement and the relevant mechanisms were discussed. Last, the perspectives for co-digestate treatment, reutilization, and disposal were summarized. Moreover, the main challenges to conductive materials amended ACoD in on-site application were proposed and the future remarks were put forward. Collectively, this review poses a scientific basis for the potential application of conductive materials in ACoD process in the future.

Enhanced methane production kinetics by graphene oxide in fed-batch tests

The study aims to prove that the addition of graphene oxide (GO) improves anaerobic digestion (AD) kinetic performance. Classical batch tests were modified to a fed-batch strategy at four GO levels while using two substrates (glucose and microcrystalline cellulose (MCC)). First-order and modified Gompertz models were respectively applied to evaluate the kinetic performance. The results showed significantly (p < 0.05) improved kinetic from the third refeeding step for both substrates. 20 mg GO per g of volatile solids (VS) led to an increase of up to 210% for the first-order rate constant (k) and up to 120% for maximum biochemical methane potential (BMP) rate (R_MAX) compared to control for glucose and MCC, respectively. The findings of this work suggest the implementation of GO in continuously operated systems to accelerate the 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.

New insights of anaerobic performance, antibiotic resistance gene removal, microbial community structure: applying graphite-based materials in wet anaerobic digestion

The addition of carbonaceous materials into anaerobic digestion (AD) has gained widespread attention due to their significant effects on anaerobic performance and antibiotic resistance gene (ARG) removal. This study selected graphite, graphene, and graphene oxide (GO) as additives to investigate variations in AD performance, ARG removal, microbial community diversity and structure in wet AD systems. The results indicated that the addition of graphite-based materials in wet AD systems could increase degradation of solid organic matters by 0.91%-3.41% and utilization of soluble organic fractions by 10.43%-13.67%, but could not stimulate methane production. After the addition of graphite and graphene, ARG removal rates were effectively increased to 90.85% and 94.22%, respectively. However, the total ARG removal rate was reduced to 77.46% with the addition of GO. In addition, the microbial diversity in the wet AD process was enhanced with the addition of GO only, graphite and graphene led to a reduction in it. As for bacterial community, graphite and graphene increased the abundance of Thermotogae from 43.43% to 57.42% and 58.74%, while GO increased the abundance of Firmicute from 49.90% to 56.27%. For the archaeal community, the proportion of hydrogenotrophic methanogens was improved when adding each graphite-based material; however, only GO increased Methanosaeta that was acetoclastic methanogens. Finally, methanogens were found as the ARG host, and ARGs that belong to the same subtype might exist in the same host bacteria.

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.

Probing the antioxidant activity of functional proteins and bioactive peptides in Hermetia illucens larvae fed with food wastes

Food waste is becoming more prevalent, and managing it is one of the most important issues in terms of food safety. In this study, functional proteins and bioactive peptides produced from the enzymatic digestion of black soldier fly (Hermetia illucens L., BSF) fed with food wastes were characterized and quantified using proteomics-based analysis. The results revealed approximately 78 peptides and 57 proteins, including 40S ribosomal protein S4, 60S ribosomal protein L8, ATP synthase subunit alpha, ribosomal protein S3, Histone H2A, NADP-glutamate dehydrogenase, Fumarate hydratase, RNA helicase, Chitin binding Peritrophin-A, Lectin C-type protein, etc. were found in BSF. Furthermore, functional analysis of the proteins revealed that the 60S ribosomal protein L5 (RpL5) in BSF interacted with a variety of ribosomal proteins and played a key role in the glycolytic process (AT14039p). Higher antioxidant activity was found in peptide sequences such as GYGFGGGAGCLSMDTGAHLNR, VVPSANRAMVGIVAGGGRIDKPILK, AGLQFPVGR, GFKDQIQDVFK, and GFKDQIQDVFK. It was concluded that the bioconversion of food wastes by BSF brought about the generation of a variety of functional proteins and bioactive peptides with strong antioxidant activity. However, more studies are required to exploit BSF's potential in the value addition of food wastes.

Modification strategies of membranes with enhanced Anti-biofouling properties for wastewater Treatment: A review

This review addresses composite membranes used for wastewater treatment, focusing heavily on the anti-biofouling properties of such membranes. Biofouling caused by the development of a thick biofilm on the membrane surface is a major issue that reduces water permeance and reduces its lifetime. Biofilm formation and adhesion are mitigated by modifying membranes with two-dimensional or zero-dimensional carbon-based nanomaterials or their modified substituents. In particular, nanomaterials based on graphene, including graphene oxide and carbon quantum dots, are mainly used as nanofillers in the membrane. Functionalization of the nanofillers with various organic ligands or compositing the nanofiller with other materials, such as silver nanoparticles, enhances the bactericidal ability of composite membranes. Moreover, such membrane modifications reduce biofilm adhesion while increasing water permeance and salt/dye rejection. This review discusses the recent literature on developing graphene oxide-based and carbon quantum dot-based composite membranes for biofouling-resistant wastewater treatment.

Metagenomic approach reveals the fates and mechanisms of antibiotic resistance genes exposed to allicins during waste activated sludge fermentation: Insight of the microbial community, cellular status and gene regulation

This work revealed the impacts of exogeneous allicins on the antibiotic resistance genes (ARGs) variations during waste activated sludge (WAS) fermentation process. The overall abundance of ARGs was respectively reduced by 4.84 and 9.42% in presence of 0.01 and 0.05 g allicin/g TSS. Allicins disrupted the EPS structure and increased the permeability of cell membranes, which resulted in the release of ARGs for subsequent removal. Allicins also reduced intracellular ATP levels, which was disadvantageous to ARGs dissemination. Besides, allicins affected the microbial community and decreased the abundance of potential hosts based on bacterial taxa-ARGs network analysis. Moreover, the metabolic pathways and genetic expressions (i.e., two-component system, quorum sensing, and SOS response) involved in ARGs propagation were down-regulated, which caused the ARGs alleviation in allicins-stressed reactors. Overall, the simultaneous responses of cellular status, bacterial host, and genetic regulation accounted for the effective ARGs reduction induced by allicins during WAS fermentation.