Impact of roxithromycin on waste activated sludge anaerobic digestion: Methane production, carbon transformation and antibiotic resistance genes

Chlorination-induced spread of antibiotic resistance genes in drinking water systems

Chlorine, the most widely utilized disinfectant for drinking water globally, has recently been implicated in facilitating the spread of antibiotic resistance genes (ARGs), raising concerns about its underestimated environmental and ecological risks. However, given the current fragmented research focus and results, a comprehensive understanding of the potential mechanisms and influencing factors behind chlorination-promoted ARGs transmission in drinking water systems is crucial. This work is the first to systematically review the variations in abundance, transmission mechanisms, influencing factors, and mitigation strategies related to ARGs during the chlorination process. The results indicated that chlorination could induce genetic mutations and promote horizontal gene transfer through multiple pathways, including increased reactive oxygen species, enhanced membrane permeability, stimulation of the SOS response, and activation of efflux pumps. In addition, this work delves into significant discoveries regarding the factors affecting ARG transmission in drinking water, such as chlorine concentration, reaction time, disinfection byproducts, pipe materials, biofilms, and the water matrix. A series of effective strategies from water source to point-of-use were proposed aimed at mitigating ARGs transmission risks in the drinking water system. Finally, we address existing challenges and outline future research directions to overcome these bottlenecks. Overall, this review aims to advance our understanding of the role of chlorination in the dissemination of ARGs and to inspire innovative research ideas for optimizing disinfection techniques, minimizing the risks of antibiotic resistance transmission, and enhancing the safety of drinking water.

Geographical distribution and risk of antibiotic resistance genes in sludge anaerobic digestion process across China

Anaerobic digestion (AD) is gaining increasing attention as the central reservoir of antibiotic resistance genes (ARGs), while the geographical distribution of ARGs in AD is neglected. Accordingly, a sampling scheme on full-scale AD plants across China was implemented, and the resistome therein was excavated. The abundance of ARGs in AD sludge ranged from 0.198 to 0.574 copies/cell. Some of the frequently reported and emergent ARGs were detected in our AD system. Both the abundance and composition of ARGs presented significant differences between the south and north regions of China, hinting the physical/economic factors may function in the formation of ARG profiles. The risk scores of AD samples were in middle of domestic and hospital wastewater. Risk scores were significantly higher in the north. Besides, the proportion of Rank I and Rank II ARGs was also higher in north, which explained the regional difference of ARG composition in a micro-perspective. This study provides a fundamental survey on the of ARG level and profile in AD process across China, reveals the biogeography of ARGs and inspires the control strategies of antibiotic resistance.

Efficient methane fermentation from the waste of a novel straw alkali-heat pretreatment-butyric acid fermentation process

ABSTRACTThe butyric acid biorefinery technology for straw is highly significant for environmental protection and the restructuring of the energy system. However, this process produces waste from alkali-heat pretreatment (PW) and butyric acid fermentation (FW). In this study, the feasibility of methane fermentation from the wastes was confirmed, with the methane production from PW and FW of 351.1 ± 11.8 and 741.5 ± 14.2 mLCH4/gVS, respectively. The initial pH and VFW/VPW of methane fermentation using the mixed waste of PW and FW were optimized at 7.5 and 1.8, respectively. The methane fermentation using the mixed waste was also verified by operating two anaerobic digesters in sequencing batch mode. At the VFW/VPW of 0.25 (actual ratio), methane production was 301.20 mLCH4/gVS with the waste load of 0.64 kgVS/m³/d. When the VFW/VPW was 1.8 (optimal ratio), methane production reached 396.45 mLCH4/gVS at the waste load of 1.20 kgVS/m3/d. This study facilitates the comprehensive utilization of all components within rice straw.

Insights into kinetic and regression models developed to estimate the abundance of antibiotic-resistant genes during biological digestion of wastewater sludge

Wastewater treatment plants are hubs of antibiotic-resistant genes (ARGs). During wastewater treatment, ARGs accumulate in wastewater sludge and some survive biological digestion. After land application of digested sludge, ARGs are transported to soil, water, and air, and may encounter humans and animals. ARGs are typically quantified by quantitative polymerase chain reaction (qPCR) on isolated DNA. Nevertheless, DNA isolation and qPCR are time-consuming, expensive, and prone to contamination. Therefore, there is a need to estimate ARGs quantities via methods that can be readily employed. Such estimation would help to protect public health via modifying biological digestion to maximize the removal of ARGs. Two approaches that make such estimation are kinetic and regression modeling. The kinetic models have been mainly of the first order. This review examines the application of the kinetic models to estimate the abundance of ARGs during biological sludge digestion. It also discusses how biological sludge digesters can be designed using kinetic models. The literature provides single and multiple regression models, from which an ARGs -Solids -Nutrients nexus, a focal point of this review, is inferred. This review demonstrates that regression models are mathematical expressions of that nexus. Also, existing challenges are highlighted and suggestions for future are provided.

Anaerobic digestion and biochar/hydrochar enhancement of antibiotic-containing wastewater: Current situation, mechanism and future prospects

The increasing consumption of antibiotics by humans and animals and their inappropriate disposal have increased antibiotic load in municipal and pharmaceutical industry waste, resulting in severe public health risks worldwide. Anaerobic digestion (AD) is the main force of antibiotic-containing wastewater treatment, and the adaptability of biochar/hydrochar (BC/HC) makes it an attractive addition to AD systems, which aim to promote methane production efficiency. Nevertheless, further studies are needed to better understand the multifaceted function of BC/HC and its role in antibiotic-containing wastewater AD. This review article examines the current status of AD of antibiotic-containing wastewater and the effects of different preparation conditions on the physicochemical properties of BC/HC and AD status. The incorporation of BC/HC into the AD process has several potential benefits, contingent upon the physical and chemical properties of BC/HC. These benefits include mitigation of antibiotic toxicity, establishment of a stable system, enrichment of functional microorganisms and enhancement of direct interspecies electron transfer. The mechanism by which BC/HC enhances the AD of antibiotic-containing wastewater, with focus on microbial enhancement, was analysed. A review of the literature revealed that the challenge of optimization and process improvement must be addressed to enhance efficiency and clarify the mechanism of BC/HC in the AD of antibiotic-containing wastewater. This review aims to provide significant insights and details into the BC/HC-enhanced AD of antibiotic-containing wastewater.

Effect, Fate and Remediation of Pharmaceuticals and Personal Care Products (PPCPs) during Anaerobic Sludge Treatment: A Review

Biomass energy recovery from sewage sludge through anaerobic treatment is vital for environmental sustainability and a circular economy. However, large amounts of pharmaceutical and personal care products (PPCPs) remain in sludge, and their interactions with microbes and enzymes would affect resource recovery. This article reviews the effects and mechanisms of PPCPs on anaerobic sludge treatment. Most PPCPs posed adverse impacts on methane production, while certain low-toxicity PPCPs could stimulate volatile fatty acids and biohydrogen accumulation. Changes in the microbial community structure and functional enzyme bioactivities were also summarized with PPCPs exposure. Notably, PPCPs such as carbamazepine could bind with the active sites of the enzyme and induce microbial stress responses. The fate of various PPCPs during anaerobic sludge treatment indicated that PPCPs featuring electron-donating groups (e.g., ·-NH2 and ·-OH), hydrophilicity, and low molecular weight were more susceptible to microbial utilization. Key biodegrading enzymes (e.g., cytochrome P450 and amidase) were crucial for PPCP degradation, although several PPCPs remain refractory to biotransformation. Therefore, remediation technologies including physical pretreatment, chemicals, bioaugmentation, and their combinations for enhancing PPCPs degradation were outlined. Among these strategies, advanced oxidation processes and combined strategies effectively removed complex and refractory PPCPs mainly by generating free radicals, providing recommendations for improving sludge detoxification.

Aptamer-free upconversion nanoparticle/silk biosensor system for low-cost and highly sensitive detection of antibiotic residues

The detection of antibiotics is crucial for safeguarding the environment, ensuring food safety, and promoting human health. However, developing a rapid, convenient, low-cost, and sensitive method for antibiotic detection presents significant challenges. Herein, an aptamer-free biosensor was successfully constructed using upconversion nanoparticles (UCNPs) coated with silk fibroin (SF), based on Förster resonance energy transfer (FRET) and the charge-transfer effect, for detecting roxithromycin (RXM). A synergistic FRET efficiency was achieved by utilizing alizarin red and RXM complexes as energy acceptors, with UCNP as the energy donor, and immobilizing an ultrathin SF protein corona within 10 nm. The biosensor detects RXM in deionized water with high sensitivity primarily through monolayer adsorption, with a detection range of 1.0 nM-141.6 nM and a detection limit as low as 0.68 nM. The performance of this biosensor was compared with the ultra-performance liquid chromatography-mass spectrometry (UPLC-MS/MS) method for detecting antibiotics in river water separately and a strong correlation between the two methods was observed. The biosensor exhibited long-term stability in aqueous solutions (up to 60 d) with no attenuation of fluorescence intensity. Furthermore, the biosensor's applicability extended to the highly sensitive detection of other antibiotics, such as azithromycin. This study introduces a low-cost, eco-friendly, and highly sensitive method for antibiotic detection, with broad potential for future applications in environmental, healthcare, and food-related fields.

Effect of Stepwise Exposure to High-Level Erythromycin on Anaerobic Digestion

High-level erythromycin (ERY) fermentation wastewater will pose serious threats to lake environments. Anaerobic digestion (AD) has advantages in treating high-level antibiotic wastewater. However, the fate of antibiotic resistance genes (ARGs) and microbial communities in AD after stepwise exposure to high-level ERY remains unclear. In this study, an AD reactor was first exposed to 0, 5, 10, 50, 100 and 200 mg/L ERY and then re-exposed to 0, 50, 200 and 500 mg/L ERY to investigate the effect of ERY on AD. The results show that AD could adapt to the presence of high-level ERY (500 mg/L) and could maintain efficient CH4 production after domestication with low-level ERY (50 mg/L). The AD process could achieve higher removal of ERY (>94%), regardless of the initial ERY concentration. ErmB and mefA, conferring resistance through target alteration and efflux pumps, respectively, were dominant in the AD process. The first exposure to ERY stimulated an increase in the total ARG abundance, while the AD process seemed to discourage ARG maintenance following re-exposure to ERY. ERY inhibited the process of acetoclastic methanogenesis, but strengthened the process of hydrogenotrophic methanogenesis. This work provides useful information for treating high-level ERY fermentation wastewater by the AD process.

Light-driven photocatalysis as an effective tool for degradation of antibiotics

Antibiotic contamination has become a severe issue and a dangerous concern to the environment because of large release of antibiotic effluent into terrestrial and aquatic ecosystems. To try and solve these issues, a plethora of research on antibiotic withdrawal has been carried out. Recently photocatalysis has received tremendous attention due to its ability to remove antibiotics from aqueous solutions in a cost-effective and environmentally friendly manner with few drawbacks compared to traditional photocatalysts. Considerable attention has been focused on developing advanced visible light-driven photocatalysts in order to address these problems. This review provides an overview of recent developments in the field of photocatalytic degradation of antibiotics, including the doping of metals and non-metals into ultraviolet light-driven photocatalysts, the formation of new semiconductor photocatalysts, the advancement of heterojunction photocatalysts, and the building of surface plasmon resonance-enhanced photocatalytic systems.

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.

Roxithromycin exposure induces motoneuron malformation and behavioral deficits of zebrafish by interfering with the differentiation of motor neuron progenitor cells

Roxithromycin (ROX), a commonly used macrolide antibiotic, is extensively employed in human medicine and livestock industries. Due to its structural stability and resistance to biological degradation, ROX persists as a resilient environmental contaminant, detectable in aquatic ecosystems and food products. However, our understanding of the potential health risks to humans from continuous ROX exposure remains limited. In this study, we used the zebrafish as a vertebrate model to explore the potential developmental toxicity of early ROX exposure, particularly focusing on its effects on locomotor functionality and CaP motoneuron development. Early exposure to ROX induces marked developmental toxicity in zebrafish embryos, significantly reducing hatching rates (n=100), body lengths (n=100), and increased malformation rates (n=100). The zebrafish embryos treated with a corresponding volume of DMSO (0.1%, v/v) served as vehicle controls (veh). Moreover, ROX exposure adversely affected the locomotive capacity of zebrafish embryos, and observations in transgenic zebrafish Tg(hb9:eGFP) revealed axonal loss in motor neurons, evident through reduced or irregular axonal lengths (n=80). Concurrently, abnormal apoptosis in ROX-exposed zebrafish embryos intensified alongside the upregulation of apoptosis-related genes (bax, bcl2, caspase-3a). Single-cell sequencing further disclosed substantial effects of ROX on genes involved in the differentiation of motor neuron progenitor cells (ngn1, olig2), axon development (cd82a, mbpa, plp1b, sema5a), and neuroimmunity (aplnrb, aplnra) in zebrafish larvae (n=30). Furthermore, the CaP motor neuron defects and behavioral deficits induced by ROX can be rescued by administering ngn1 agonist (n=80). In summary, ROX exposure leads to early-life abnormalities in zebrafish motor neurons and locomotor behavior by hindering the differentiation of motor neuron progenitor cells and inducing abnormal apoptosis.

Performance of sewage sludge treatment for the removal of antibiotic resistance genes: Status and prospects

Wastewater treatment plants (WWTPs) receive wastewater containing antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs), which are predominant contributors to environmental pollution in water and soil. Of these sources, sludge is a more significant contributor than effluent. Knowing how sludge treatment affects the fate of ARGs is vital for managing the risk of these genes in both human and natural environments. This review therefore discusses the sources and transmission of ARGs in the environment and highlights the risks of ARGs in sludge. The effects of co-existing constituents (heavy metals, microplastics, etc.) on sludge and ARGs during treatment are collated to highlight the difficulty of treating sludge with complex constituents in ARGs. The effects of various sludge treatment methods on the abundances of ARGs in sludge and in soil from land application of treated sludge are discussed, pointing out that the choice of sludge treatment method should take into account various potential factors, such as soil and soil biology in subsequent land application. This review offers significant insights and explores the abundances of ARGs throughout the process of sludge treatment and disposal. Unintentional addition of antibiotic residues, heavy metals, microplastics and organic matter in sludge could significantly increase the abundance and reduce the removal efficiency of ARGs during treatment, which undoubtedly adds a barrier to the removal of ARGs from sludge treatment. The complexity of the sludge composition and the diversities of ARGs have led to the fact that no effective sludge treatment method has so far been able to completely eliminate the ecological risk of ARGs. In order to reduce risks resulting by transmission of ARGs, technical and management measures need to be implemented.

Comprehending the impact of berberine on anaerobic digestion of waste activated sludge

Berberine is a natural isoquinoline alkaloid performing wide-spectrum antimicrobial and antiviral effects like antibiotics. Its production generates berberine containing wastewater, and berberine adsorbed on waste activated sludge (WAS) will unavoidably enter the anaerobic digestion (AD) system while its impact on the AD process is unknown. Our research found that berberine of 20 mg/L (BBR20) slightly enhanced the methane yield (4.2 ± 0.6%) under mesophilic condition (35.0 ± 1.0 °C). However, 100 and 500 mg/L (BBR100 and BBR500) depressed methane production by 17.3 ± 4.3% and 83.2 ± 0.4%; meanwhile more soluble chemical oxygen demand (SCOD) including volatile fatty acid (VFA), protein, and polysaccharide were left in the fermentation broth, which led to an increase in sludge reduction. 88.3 ± 0.09%-99.1 ± 0.04% of berberine was distributed in the sludge phase and could be efficiently removed even under a high berberine level of 500 mg/L during the AD process. Exposure to different berberine concentrations promoted sludge dissolution and triggered more sludge extracellular polymeric substances (EPS) being dissolved. Lower berberine concentration (20 mg/L) enhanced acidification and methanogenesis steps, resulting in a final methane generation increase. While hydrolysis, acidification and methanogenesis processes were all inhibited by 100 and 500 mg/L berberine. Microbial analysis revealed that the main acid-producing bacteria genera were changed as Bacteroidetes vadinHA17 dominated in control, BBR20 and BBR100 groups, was replaced by Petrimonas in BBR500. Additionally, Methanosaeta, as a strict acetoclastic methanogen, was suppressed under exposure to 100 and 500 mg/L berberine. Accordingly, the declined abundance of archaea genera consuming acetic acid caused more VFA accumulation and less methane production in BBR100 and BBR500 groups.

Performance of volatile fatty acids production from food waste at the presence of alkyl ethoxy polyglycosides and sodium dodecyl sulfate

In the current context of technological and industrial development, strategies for sustainable development and resource utilization have become increasingly important. FW anaerobic fermentation (Fermentation of Wastes) is a process that utilizes organic waste for biotransformation and is widely used for the production of volatile fatty acids (VFAs). Volatile fatty acids (VFAs) are a kind of high value-added product generated from anaerobic fermentation process, and has extensive applications in chemical synthesis and electricity generation. This study investigated the performance of VFAs production from food waste at the presence of alkyl ethoxy polyglycosides (AEG) and sodium dodecyl sulfate (SDS). The highest yield of VFAs was obtained at 0.1 g AEG/g TS (14.53 g COD/L), which increased by 25.80% than the Blank. But inhibited phenomenon was observed at other reactors with relatively low yield and delayed fermentation time. The inhibition of lactate's production and bioconversion delayed the fermentation time, and SDS has changed the acidogenic fermentation type from lactate-butyrate fermentation to acetate fermentation. In addition, more organic matter dissolved in the fermentation liquor with the addition of AEG and SDS, but the hydrolysis and acidification of polysaccharide were inhibited to some extent. Microbial community analysis showed that the abundance of key bacteria Clostridium has significantly decreased from 82.71% (Blank) to 33.54% (AEG) and 23.72% (SDS), leading to low VFAs production performance.

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.

Dose-response and type-dependent effects of antiviral drugs in anaerobic digestion of waste-activated sludge for biogas production

In the context of the COVID-19 pandemic, antiviral drugs (AVDs) were heavily excreted into wastewater and subsequently enriched in sewage sludge due to their widespread use. The potential ecological risks of AVDs have attracted increasing attention, but information on the effects of AVDs on sludge anaerobic digestion (AD) is limited. In this study, two typical AVDs (lamivudine and ritonavir) were selected to investigate the responses of AD to AVDs by biochemical methane potential tests. The results indicated that the effects of AVDs on methane production from sludge AD were dose- and type-dependent. The increased ritonavir concentration (0.05-50 mg/kg TS) contributed to an 11.27-49.43% increase in methane production compared with the control. However, methane production was significantly decreased at high lamivudine doses (50 mg/kg TS). Correspondingly, bacteria related to acidification were affected when exposed to lamivudine and ritonavir. Acetoclastic and hydrotropic methanogens were inhibited at a high lamivudine dose, while ritonavir enriched methylotrophic and hydrotropic methanogens. Based on the analysis of intermediate metabolites, the inhibition of lamivudine and the promotion of ritonavir on acidification and methanation were confirmed. In addition, the existence of AVDs could affect sludge properties. Sludge solubilization was inhibited when exposed to lamivudine and enhanced by ritonavir, perhaps caused by their different structures and physicochemical properties. Moreover, lamivudine and ritonavir could be partially degraded by AD, but 50.2-68.8% of AVDs remained in digested sludge, implying environmental risks.

Effects of antibiotics on anaerobic digestion of sewage sludge: Performance of anaerobic digestion and structure of the microbial community

As a common biological engineering technology, anaerobic digestion can stabilize sewage sludge and convert the carbon compounds into renewable energy (i.e., methane). However, anaerobic digestion of sewage sludge is severely affected by antibiotics. This review summarizes the effects of different antibiotics on anaerobic digestion of sewage sludge, including production of methane and volatile fatty acids (VFAs), and discusses the impact of antibiotics on biotransformation processes (solubilization, hydrolysis, acidification, acetogenesis and methanogenesis). Moreover, the effects of different antibiotics on microbial community structure (bacteria and archaea) were determined. Most of the research results showed that antibiotics at environmentally relevant concentrations can reduce biogas production mainly by inhibiting methanogenic processes, that is, methanogenic archaea activity, while a few antibiotics can improve biogas production. Moreover, the combination of multiple environmental concentrations of antibiotics inhibited the efficiency of methane production from sludge anaerobic digestion. In addition, some lab-scale pretreatment methods (e.g., ozone, ultrasonic combined ozone, zero-valent iron, Fe³⁺ and magnetite) can promote the performance of anaerobic digestion of sewage sludge inhibited by antibiotics.

Effects of heavy metals and antibiotics on performances and mechanisms of anaerobic digestion

Anaerobic digestion (AD) is an efficacious technology to recover energy from organic wastes/wastewater, while the efficiency of AD could be limited by metals and antibiotics in substrates. It is of great significance to deeply understand the interaction mechanisms of metals and antibiotics with anaerobic microorganisms, as well as the combined effects of metals and antibiotics, which will help us break the inherent dysfunction of AD system and promote the efficient operation of AD. Therefore, this paper reviews the effects of metals, antibiotics and their combinations on AD performance, as well as the combined effects and interactional mechanisms of metals and antibiotics with anaerobic microorganisms. In addition, control strategies and future research needs are proposed. This review provides valuable information for the enhancement strategies and engineering applications of AD for organic wastes/wastewater containing metals and antibiotics.

Antibiotics in anaerobic digestion: Investigative studies on digester performance and microbial diversity

The ever-increasing consumption of antibiotics in both humans and animals has increased their load in municipal and pharmaceutical industry waste and may cause serious damage to the environment. Impact of antibiotics on the performance of commercially used anaerobic digesters in terms of bioenergy output, antibiotics' removal and COD removal have been compared critically with a few studies indicating >90% removal of antibiotics. AnMBR performed the best in terms of antibiotic removal, COD removal and methane yield. Most of the antibiotics investigated have adverse effects on microbiome associated with different stages and methane generation pathways of AD which has been assessed using high throughput technologies like metatranscriptomics, metaproteomics and flow cytometry. Perspectives have been given for understanding the fate and elimination of antibiotics from AD. The challenge of optimization and process improvement needs to be addressed to increase efficiency of the anaerobic digesters.

Advances in pretreatment strategies to enhance the biodegradability of waste activated sludge for the conversion of refractory substances

Anaerobic digestion (AD) is a low-cost technology widely used to divert waste activated sludge (WAS) to renewable energy production, but is generally restricted by its poor biodegradability which mainly caused by the endogenous and exogenous refractory substances present in WAS. Several conventional methods such as thermal-, chemical-, and mechanical-based pretreatment have been demonstrated to be effective on organics release, but their functions on refractory substances conversion are overlooked. This paper firstly reviewed the presence and role of endogenous and exogenous refractory substances in anaerobic biodegradability of WAS, especially on their inhibition mechanisms. Then, the pretreatment strategies developed for enhancing WAS biodegradability by facilitating refractory substances conversion were comprehensively reviewed, with the conversion pathways and underlying mechanisms being emphasized. Finally, the future research needs were directed, which are supposed to improve the circular bioeconomy of WAS management from the point of removing the hindering barrier of refractory substances on WAS biodegradability.

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.

Efficient removal of antibiotics from water resources is a public health priority: a critical assessment of the efficacy of some remediation strategies for antibiotics in water

This review discusses the fundamental principles and mechanism of antibiotic removal from water of some commonly applied treatment techniques including chlorination, ozonation, UV-irradiation, Fenton processes, photocatalysis, electrochemical-oxidation, plasma, biochar, anaerobicdigestion, activated carbon and nanomaterials. Some experimental shortfalls identified by researchers such as certain characteristics of degradation agent applied and the strategies explored to override the identified limitations are briefly discussed. Depending on interactions of a range of factors including the type of antibiotic compound, operational parameters applied such as pH, temperature and treatment time, among other factors, all reviewed techniques can eliminate or reduce the levels of antibiotic compounds in water to varying extents. Some of the reviewed techniques such as anaerobic digestion generally require longer treatment times (up to 360, 193 and 170 days, according to some studies), while others such as photocatalysis achieved degradation within short contact time (within a minimum of 30, but up to 60, 240, 300 and 1880 minutes, in some cases). For some treatment techniques such as ozonation and Fenton, it is apparent that subjecting compounds to longer treatment times may improve elimination efficiency, whereas for some other techniques such as nanotechnology, application of longer treatment time generally meant comparatively minimal elimination efficiency. Based on the findings of experimental studies summarized, it is apparent that operational parameters such as pH and treatment time, while critical, do not exert sole or primary influence on the elimination percentage(s) achieved. Elimination efficiency achieved rather seems to be due more to the force of a combination of several factors.

An innovative strategy to enhance the ensiling quality and methane production of excessively wilted wheat straw: Using acetic acid or hetero-fermentative lactic acid bacterial community as additives

Ensiling is an effective storage strategy for agricultural biomass, especially for energy crops (mainly energy grasses and maize). However, the ensiling of excessively wilted crop straw is limited due to material characteristics, such as a high lignocellulosic content and low water-soluble carbohydrate and moisture contents. In this study, acetic acid or hetero-fermentative lactic acid bacterial community (hetero-fermentative LAB) were employed as silage additives to improve the ensiling process of excessively wilted wheat straw (EWS). The results showed that the additives inhibited the growth of Enterobacteriaceae and Clostridium_sensu_stricto_12, whose abundances decreased from 55.8% to 0.03-0.2%, respectively. The growth of Lactobacillus was accelerated, and the abundances increased from 1.3% to 80.1-98.4% during the ensiling process. Lactic acid fermentation was the dominant metabolic pathway in the no additive treatment. The additives increased acetic acid fermentation and preserved the hemicellulose and cellulose contents, increasing the methane yield by 17.7-23.9%. This study shows that ensiling with acetic acid or hetero-fermentative LAB is an effective preservation and storage strategy for efficient methane production from EWS.

Migration of antibiotic resistance genes and evolution of flora structure in the Xenopus tropicalis intestinal tract with combined exposure to roxithromycin and oxytetracycline

The intestinal flora is one of the most important environments for antibiotic resistance development, owing to its diverse mix of bacteria. An excellent medicine model organism, Xenopus tropicalis, was selected to investigate the spread of antibiotic resistance genes (ARGs) in the intestinal bacterial community with single or combined exposure to roxithromycin (ROX) and oxytetracycline (OTC). Seventeen resistance genes (tetA, tetB, tetE, tetM, tetO, tetS, tetX, ermF, msrA, mefA, ereA, ereB, mphA, mphB, intI1, intI2, intI3) were detected in the intestines of Xenopus tropicalis living in three testing tanks (ROX tanks, OTC tanks, ROX + OTC tanks) and a blank tank for 20 days. The results showed that the relative abundance of total ARGs increased obviously in the tank with single stress but decreased in the tank with combined stress, and the genes encoding the macrolide antibiotic efflux pump (msrA), phosphatase (mphB) and integron (intI2, intI3) were the most sensitive. With the aid of AFM scanning, DNA was found to be scattered short chain in the blank, became extended or curled and then compacted with the stress from a single antibiotic, and was compacted and then fragmented with combined stress, which might be the reason for the variation of the abundance of ARGs with stress. The ratio of Firmicutes/Bacteroides related to diseases was increased by ROX and OTC. The very significant correlation between intI2 and intI3 with tetS (p ≤ 0.001) hinted at a high risk of ARG transmission in the intestines. Collectively, our results suggested that the relative abundance of intestinal ARGs could be changed depending on the intestinal microbiome and DNA structures upon exposure to antibiotics at environmental concentrations.

Migration, Transformation and Removal of Macrolide Antibiotics in The Environment: A Review

Macrolide antibiotics (MAs), as a typical emerging pollutant, are widely detected in environmental media. When entering the environment, MAs can interfere with the growth, development and reproduction of organisms, which has attracted extensive attention. However, there are few reviews on the occurrence characteristics, migration and transformation law, ecotoxicity and related removal technologies of MAs in the environment. In this work, combined with the existing relevant research, the migration and transformation law and ecotoxicity characteristics of MAs in the environment are summarized, and the removal mechanism of MAs is clarified. Currently, most studies on MAs are based on laboratory simulation experiments, and there are few studies on the migration and transformation mechanism between multiphase states. In addition, the cost of MAs removal technology is not satisfactory. Therefore, the following suggestions are put forward for the future research direction. The migration and transformation process of MAs between multiphase states (such as soil-water-sediment) should be focused on. Apart from exploring the new treatment technology of MAs, the upgrading and coupling of existing MAs removal technologies to meet emission standards and reduce costs should also be concerned. This review provides some theoretical basis and data support for understanding the occurrence characteristics, ecotoxicity and removal mechanism of MAs.

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.

Long-Term, Simultaneous Impact of Antimicrobials on the Efficiency of Anaerobic Digestion of Sewage Sludge and Changes in the Microbial Community

The aim of this study was to evaluate the influence of simultaneous, long-term exposure to increasing concentrations of three classes of antimicrobials (β-lactams, fluoroquinolones and nitroimidazoles) on: (1) the efficiency of anaerobic digestion of sewage sludge, (2) qualitative and quantitative changes in microbial consortia that participate in methane fermentation, and (3) fate of antibiotic resistance genes (ARGs). Long-term supplementation of sewage sludge with a combination of metronidazole, amoxicillin and ciprofloxacin applied at different doses did not induce significant changes in process parameters, including the concentrations of volatile fatty acids (VFAs), or the total abundance of ARGs. Exposure to antibiotics significantly decreased methane production and modified microbial composition. The sequencing analysis revealed that the abundance of OTUs characteristic of Archaea was not correlated with the biogas production efficiency. The study also demonstrated that the hydrogen-dependent pathway of methylotrophic methanogenesis could significantly contribute to the stability of anaerobic digestion in the presence of antimicrobials. The greatest changes in microbial biodiversity were noted in substrate samples exposed to the highest dose of the tested antibiotics, relative to control. The widespread use of antimicrobials increases antibiotic concentrations in sewage sludge, which may decrease the efficiency of anaerobic digestion, and contribute to the spread of antibiotic resistance (AR).

Enhancing sulfide mitigation via the sustainable supply of oxygen from air-nanobubbles in gravity sewers

Traditional air or oxygen injection is an effective and economical mitigation strategy for sulfide control in pressure sewers, but it is not suitable for gravity sewers due to the low solubility of oxygen in water under normal atmospheric conditions. Herein, an air-nanobubble (ANB) injection method was proposed for sulfide mitigation in gravity sewers, and its sulfide control efficiency was evaluated by long-term laboratory gravity sewer reactors. The results showed that an average inhibition rate of 45.36% for sulfide was obtained when ANBs were implemented, which was 3.75 times higher than that of the traditional air injection method, revealing the effectiveness and feasibility of the ANB injection method. As suggested by microbial community analysis of sewer biofilms, the relative abundance of sulfate-reducing bacteria (SRB) decreased 40.57% while that of sulfur oxidizing bacteria (SOB) increased 215.27% in the presence of ANBs, indicating that sulfide mitigation by ANB injection included both the inhibition of sulfide production and the oxidation of dissolved sulfide. The specific cost consumption of ANB injection was 1.7 $/kg-S, which was only 6.85% of that of traditional air injection (24.8 $/kg-S), suggesting that the sustainable supply of oxygen based on ANB injection is not only environmentally but also economically beneficial for sulfide mitigation. The findings of this study may provide an efficient sulfide mitigation strategy for the management of corrosion and malodour issues in the poorly ventilated gravity sewers.

Perfluorooctanoic acid triggers oxidative stress in anaerobic digestion of sewage sludge

Perfluorooctanoic acid (PFOA), as a recalcitrant organic pollutant, inevitably enters wastewater treatment facilities and is enriched in settled sludge. However, the potential impact of PFOA on sludge treatment has never been documented. In this study, the effect of PFOA on anaerobic digestion of sewage sludge and its underlying mechanism were investigated through batch and long-term experiments. The presence of PFOA was found to be deleterious for methane production from sewage sludge. 170 mg/kg total solids PFOA reduced the cumulative methane production from 197.1 ± 1.92-159.9 ± 3.10 mL/g volatile solids. PFOA induces the production of reactive oxygen species, which directly leads to cell inactivation and interferes with methane production. PFOA stimulates microorganisms to secrete more extracellular polymeric substances (mainly proteins), which not only hinders the solubilization of organic matter but also down-regulate enzyme activities to inhibit acidification and methanogenesis. In addition, PFOA reduces the diversity of microorganisms, especially the abundance of acid-producing bacteria and methanogens, making the microbial community unfavorable for methane production.

Erythromycin stimulates rather than inhibits methane production in anaerobic digestion of antibiotic fermentation dregs

Erythromycin fermentation dregs (EFD) as one kind of organic-rich biosolid was of great potential for methane production. However, the influence of residual erythromycin (ERY) on the anaerobic digestion process of EFD remains unclear. In this study, a batch test was conducted with different ERY concentrations to investigate its effects on methanogenesis. The antibiotic resistance genes and microbial community composition were analyzed to explore the potential mechanism. The results showed that more than 80% of ERY was removed after 30 days digestion. Furthermore, 100, 200 and 300 mg/L of ERY presented no significant effect on the performance of anaerobic digestion. Instead, a high concentration of ERY (500 mg/L) increased 13% rather than inhibited the methane yields. Moreover, the proliferation of the methylase gene (e.g., ermA/T) was promoted under the high pressure of ERY. The relative abundance of acetogenic bacteria (Sedimentibacter) and mixotrophic archaea (Methanosarcina) were enhanced, indicating that their syntrophic association would play the dominant role in the stimulating effects of methanogenesis.

Metatranscriptomic insight into the effects of antibiotic exposure on performance during anaerobic co-digestion of food waste and sludge

Antibiotics are inevitably entered into anaerobic co-digestion (AcoD) system of food waste (FW) and sludge along with the addition of abundant antibiotic-containing activated sludge. However, the in-depth insights into antibiotics affecting AcoD performance have not comprehensively studied. In present study, the results showed that tetracycline (TC), sulfamethoxazole (SMZ) and erythromycin (ERY) inhibited and delayed methane production except for 5 mg/L ERY. By comparison, TC and SMZ significantly inhibited the cumulative methane yields (one-way ANOVA, p < 0.01), and the inhibition effects were magnified as the antibiotic level increased. Physicochemical and methane yield analysis indicated antibiotics inhibited hydrolysis process and delayed methanogenesis process, which was in line with the declined abundance of acetogenic Proteiniphilum and hydrogenotrophic Methanobacterium during AcoD. Furthermore, metatranscriptomic analysis demonstrated the microbial activities of major organic and energy metabolism were down-regulated under antibiotics exposure, thereby down-regulating the expressions of key coenzymes (coenzymes M, F420, methanofuran) biosynthesis for methanogenesis and methane metabolism. The declined methanogenesis activity was completely consistent with the inhibited activity of dominant Methanosarcina and methane production, proving the importance of Methanosarcina on methane production. This study provides new metatranscriptomic evidence into the effects of antibiotics on methanogenesis during AcoD.

Calcium peroxide significantly enhances volatile solids destruction in aerobic sludge digestion through improving sludge biodegradability

This work put up a novel strategy of applying calcium peroxide (CaO₂) in aerobic sludge digestion and provided insights into such system. The degradation percentage of sludge and total inorganic nitrogen production in the digesters with CaO₂ at 0.02 g/g-VS-WAS increased by 25.8% and 18.8% of control. CaO₂ addition allowed various key microbes related to organics degradation to accumulate in the system. Moreover, the modelling and chemical (i.e., excitation emission matrix (EEM) fluorescence and fourier transformation spectroscopy (FTIR)) analyses revealed that CaO₂ addition enhanced sludge biodegradability with more release of biodegradable organics and increased degradation of recalcitrant organics, which can be transformed into biodegradable organics with the action of CaO₂. Subsequent transformation test indicated that CaO₂ enabled to promote hydrolysis and catabolism of biodegradable substrates in sludge. Further investigations on function mechanism suggested that CaO₂ carried on positive action for sludge aerobic digestion mainly through the enhancement by ·OH.

Responses of methane production, microbial community and antibiotic resistance genes to the mixing ratio of gentamicin mycelial residues and wheat straw in anaerobic co-digestion process

Anaerobic co-digestion (AcoD) of gentamicin mycelial residues (GMRs), a kind of nitrogen-rich biowaste, and wheat straw (WS) is an attractive technology for the recycling of GMRs. However, the effects of the co-substrate ratio on methane production, system stability and antimicrobial resistance during co-digestion remain unclear. Thus, this study aimed to fill in the blanks through AcoD of GMRs and WS with different mixing ratios (1:0, 2:1, 1:1, 1:2, 0:1, VS basis) via batch tests. Results showed that AcoD facilitated methane production than mono anaerobic digestion and reduced the accumulation of the toxic substances, such as ammonia nitrogen and humic-like substances. The maximum methane production was obtained at the reactors with the mixing ratio of 1:1 and 1:2 (R-1:1 and R-1:2), which matched with the relative abundance of key enzymes related to methanogenesis predicted by PICRUSt. Microbial community analysis indicated that Methanosaeta was the most dominant methanogen in the AcoD reactors. The highest relative abundance of Methanosaeta (45.1%) was obtained at R-1:1 due to the appropriate AcoD conditions, thus, providing greater possibilities for high stability of AcoD system. Additionally, AcoD of the GMRs and WS under the mixing ratio of 1:1 and 1:2 did not prompt the increase of antibiotic resistance genes (ARGs). Not only that, the likelihood of horizontal gene transfer declined in R-1:1 due to the weaker connection and transport between host and recipient bacteria. Findings of this study suggested that the suitable mixing ratio of GMRs and WS contributes to methane production and system stability, and reduces the dissemination risks of ARGs.

Biochar for agronomy, animal farming, anaerobic digestion, composting, water treatment, soil remediation, construction, energy storage, and carbon sequestration: a review

In the context of climate change and the circular economy, biochar has recently found many applications in various sectors as a versatile and recycled material. Here, we review application of biochar-based for carbon sink, covering agronomy, animal farming, anaerobic digestion, composting, environmental remediation, construction, and energy storage. The ultimate storage reservoirs for biochar are soils, civil infrastructure, and landfills. Biochar-based fertilisers, which combine traditional fertilisers with biochar as a nutrient carrier, are promising in agronomy. The use of biochar as a feed additive for animals shows benefits in terms of animal growth, gut microbiota, reduced enteric methane production, egg yield, and endo-toxicant mitigation. Biochar enhances anaerobic digestion operations, primarily for biogas generation and upgrading, performance and sustainability, and the mitigation of inhibitory impurities. In composts, biochar controls the release of greenhouse gases and enhances microbial activity. Co-composted biochar improves soil properties and enhances crop productivity. Pristine and engineered biochar can also be employed for water and soil remediation to remove pollutants. In construction, biochar can be added to cement or asphalt, thus conferring structural and functional advantages. Incorporating biochar in biocomposites improves insulation, electromagnetic radiation protection and moisture control. Finally, synthesising biochar-based materials for energy storage applications requires additional functionalisation.

Dissemination of antibiotic resistance under antibiotics pressure during anaerobic co-digestion of food waste and sludge: Insights of driving factors, genetic expression, and regulation mechanism

This study revealed the effects and regulation mechanisms on antibiotic resistance genes (ARGs) dissemination during anaerobic co-digestion (AcoD) of food waste and sludge under the exposure of tetracycline, sulfamethoxazole (SMZ) and erythromycin (ERY). Results indicated antibiotics significantly increased the abundance of ARGs, and selectively enriched integron gene, suggesting antibiotics promoted the dissemination of ARGs. Procrustes analysis indicated that bacterial community, integrons and physicochemical properties displayed significant correlations with ARGs, and they respectively contributed 10.61%, 6.94% and 2.97% of explanations on ARGs variation. Especially, the maximum combined contribution (48.6%) of bacterial community and integrons, implying their significances on ARGs alteration. Metatranscriptomic analysis further demonstrated antibiotics upregulated the expressions of total ARGs and virulence factors, raising potential risks. The proposed mechanisms for ARGs dissemination facilitated by antibiotics might be attributed to the changes of ARGs-regulated functions for inducing DNA/cell damage and DNA conjugation during AcoD.

Biochar amendment to advance contaminant removal in anaerobic digestion of organic solid wastes: A review

Anaerobic digestion (AD) has been widely applied to convert organic solid wastes into biogas, a renewable energy, and digestate, a bio-fertilizer, to sustain waste management. Nevertheless, several vexing contaminants in OSWs restrict digestate application in agriculture. Biochar has been evidenced to effectively improve AD by promoting organic biodegradation and alleviating the accumulation of inhibitory substances (e.g. ammonia and volatile fatty acids). Furthermore, biochar could advance contaminant removal in AD given its highly porous, conductive and alkaline features. Thus, this review aims to highlight the role of biochar amendment to advance contaminant removal in AD of OSWs. Key contaminants, such as antibiotics, heavy metals, microplastics, polycyclic aromatic hydrocarbons, furfural and 5-hydroxy methyl furfural (5-HMF) that ubiquitously present in OSWs were demonstrated. The underlying mechanisms of biochar to amend the removal of these contaminants by AD were discussed. Furthermore, future perspectives to the development of biochar-assisted AD for OSWs treatment were provided.

Response of microbial communities based on full-scale classification and antibiotic resistance genes to azithromycin and copper combined pollution in activated sludge nitrification laboratory mesocosms at low temperature

This study aimed to investigate the short-term response of abundant-rare genera and antibiotic resistance genes (ARGs) to azithromycin (AZM, 0.05-40 mg/L) and copper (1 mg/L) combined pollution in activated sludge nitrification system at low temperature. Nitrification was as expected inhibited in stress- and post-effects periods under AZM concentration higher than 5 mg/L. Abundant and rare taxa presented dissimilar responses based on full-scale classification. Conditionally rare or abundant taxa (CRAT) were keystone taxa. Relative abundance of ammonia-oxidizing archaea increased, and three aerobic denitrifying bacteria (Brevundimonas, Comamonas and Trichococcus) were enriched (from 9.83% to 68.91% in total). Ammonia nitrogen assimilating into Org-N and denitrification may be nitrogen pathways based on predict analysis. 29 ARGs were found with more co-occurrence patterns and high concentration of AZM (greater than 5 mg/L) caused their proliferation. Importantly, expect for some abundant taxa, rare taxa, potential pathogens and nitrogen-removal functional genera were the main potential hosts of ARGs.

Responses of anaerobic digestion of waste activated sludge to long-term stress of benzalkonium chlorides: Insights to extracellular polymeric substances and microbial communities

Quaternary ammonium compounds have gained widespread attention due to their extensive enrichment in waste activated sludge (WAS) and potentially adverse effect to anaerobes. This study selected benzalkonium chlorides (BACs) as model to reveal the responses of anaerobic digestion of WAS to long-term stress of BACs. Results showed that the solubilization enhancement of WAS contributed by BACs was the acceleration of cell lysis, rather than the disruption of extracellular polymeric substances, and the accumulation improvement of short chain fatty acids (SCFAs) attributed to hydrolysis improvement and methanogenesis inhibition at either medium -or high level of BACs. In addition, a low level had no significant effect on the production of methane compared to control, with averages of 0.059 and 0.055 m³/(m³·d), respectively, whereas a medium level reduced methane production to 20% of control, and a high level almost completely inhibited methanogenesis. Correspondingly, BACs could shift microbial communities related to SCFAs and methane productions. For the bacterial community, a high level of BACs led to abundance reductions of Firmicutes, Bacteroidetes, Acidobacteria and Chloroflexi, but Synergistetes was increased to 10.5%, which was almost not detected either in control or at a low level of BACs. And for dominant archaeal community, they tended to be shifted from acetotrophic to hydrogenotrophic methanogens with BACs increasing from low to high level. These findings provided some new insights for the role of BACs in anaerobic digestion, as well as resource recovery from WAS.

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

This study explored and compared the influence of graphite, graphene, and graphene oxide (GO) on the performance of anaerobic co-digestion fed with sewage sludge and food waste, the variations of antibiotic resistance genes (ARGs), and the evolution of microbial community. Graphene exhibited the best performance for improving methane production and organic degradation, which increased by 36.09% and 23.07% compared with control group. The experimental results showed that graphene had the greatest influence on the removal efficiency of blaOXA-1, macrolide resistance genes (ermF and ermB), and some tetracycline resistance genes (tetQ and tetX); however, the removal efficiency of sulfonamide resistance genes (sul1 and sul2), intI1, and some tetracycline resistance genes (tetM, tetO, and tetW) were highest when GO was added. Network analysis indicated that the host cells of mefA, ermB, and tetO were different from other ARG host cells; moreover, graphene controlled the horizontal transfer of ARGs between microbial communities.

Comprehensive role of thermal combined ultrasonic pre-treatment in sewage sludge disposal

Thermal/ultrasonic pre-treatment of sludge has been proven to break the hydrolysis barriers of sewage sludge (SS) and improve the performance of anaerobic digestion (AD). In this study, the objective was to investigate whether the combination of two pre-treatment methods can achieve better results on the AD of SS. The results indicated that, compared with the control group and separate pre-treatment groups, the thermal combined ultrasonic pre-treatment presented more obvious solubilization of soluble proteins, polysaccharides, and other organic matters in SS. The combined method promoted the dissolution of protein-like substances more effectively, with biogas production increased by 19% and the volatile solid (VS) removal rate improved to above 50% compared with the control group. The capillary suction time is reduced by about 85%, which greatly improved the dewatering performance of SS. In addition, the combined method has advantages in degrading sulfonamide antibiotics, roxithromycin and tetracycline. Particularly, by analyzing the interaction between the degradation of different antibiotics and the composition of dissolved organic matters (DOMs), it was found that the composition of DOMs could affect the degradability of different antibiotics. Among them, the high content of tyrosine-like and tryptophan-like was conducive to the degradation of sulfamethoxazole, and the high content of fulvic acid-like and humic acid-like was conducive to the degradation of roxithromycin and tetracycline. This work evaluated the comprehensive effect of thermal combined ultrasonic pre-treatment on SS disposal and provided useful information for its engineering.

Engineering anaerobic digestion via optimizing microbial community: effects of bactericidal agents, quorum sensing inhibitors, and inorganic materials

Anaerobic digestion of sewage sludge (SS) is one of the effective ways to reduce the waste generated from human life activities. To date, there are many reports to improve or repress methane production during the anaerobic digestion of SS. In the anaerobic digestion process, many microorganisms work positively or negatively, and as a result of their microbe-to-microbe interaction and regulation, methane production increases or decreases. In other words, understanding the complex control mechanism among the microorganisms and identifying the strains that are key to increase or decrease methane production are important for promoting the advanced production of bioenergy and beneficial compounds. In this mini-review, the literature on methane production in anaerobic digestion has been summarized based on the results of antibiotic addition, quorum sensing control, and inorganic substance addition. By optimizing the activity of microbial groups in SS, methane or acetate can be highly produced. KEY POINTS: • Bactericidal agents such as an antibiotic alter microbial community for enhanced CH₄ production. • Bacterial interaction via quorum sensing is one of the key points for biofilm and methane production. • Anaerobic digestion can be altered in the presence of several inorganic materials.

Perfluorooctane sulfonate decreases the performance of a sequencing batch reactor system and changes the sludge microbial community

The existence of perfluorooctane sulfonate (PFOS) in large quantities threatens environment biosafety. However, the fate of PFOS in a sequencing batch reactor (SBR) system and its influence in system has not yet been revealed. In this study, the fate and behavior of PFOS in an SBR processing system were investigated. Mass balance analyses revealed that PFOS removal was mainly through adsorption. After the reactors were run for 20 days, the PFOS (100 mg/L) removal rate was only 28%. Under the influence of PFOS, the removal rates of chemical oxygen demand (COD) and ammonia nitrogen dropped rapidly from 92, 98% to 23, 35% in the 20th day of system operation, respectively, while, accumulation of nitrite and nitrate was reduced. Compared with the control group, PFOS stimulates microorganisms to secrete more soluble microbial products (SMP) and extracellular polymeric substances (EPS). The adsorption of PFOS and EPS causes sludge bulking and decreases settling. The richness and diversity of microorganisms decreased significantly, affecting the system's removal of COD and ammonia nitrogen. Therefore, the SBR system is not suitable for treating wastewater containing PFOS. It is necessary to remove PFOS through pretreatment to reduce its impact on the SBR system.

The impact of antimicrobials on the efficiency of methane fermentation of sewage sludge, changes in microbial biodiversity and the spread of antibiotic resistance

The study was designed to simultaneously evaluate the influence of high doses (512-1024 µg/g) the most commonly prescribed antimicrobials on the efficiency of anaerobic digestion of sewage sludge, qualitative and quantitative changes in microbial consortia responsible for the fermentation process, the presence of methanogenic microorganisms, and the fate of antibiotic resistance genes (ARGs). The efficiency of antibiotic degradation during anaerobic treatment was also determined. Metronidazole, amoxicillin and ciprofloxacin exerted the greatest effect on methane fermentation by decreasing its efficiency. Metronidazole, amoxicillin, cefuroxime and sulfamethoxazole were degraded in 100%, whereas ciprofloxacin and nalidixic acid were least susceptible to degradation. The most extensive changes in the structure of digestate microbiota were observed in sewage sludge exposed to metronidazole, where a decrease in the percentage of bacteria of the phylum Bacteroidetes led to an increase in the proportions of bacteria of the phyla Firmicutes and Proteobacteria. The results of the analysis examining changes in the concentration of the functional methanogen gene (mcrA) did not reflect the actual efficiency of methane fermentation. In sewage sludge exposed to antimicrobials, a significant increase was noted in the concentrations of β-lactam, tetracycline and fluoroquinolone ARGs and integrase genes, but selective pressure was not specific to the corresponding ARGs.

Inhibition of tetracycline on anaerobic digestion of swine wastewater

The inhibition of tetracycline on anaerobic digestion of synthetic swine wastewater was examined with a semi-continuous operation for 103 days at a dosage ranging 2-8 mg/L. COD concentrations, VFA compositions in effluents and methane production were measured. The negative effects of tetracycline on the four individual steps of anaerobic digestion and its toxicity on anaerobic microorganisms were also evaluated. Results showed that continuous addition of 8 mg/L tetracycline in the bioreactor resulted in 73.28% reduction of daily methane production and made anaerobic digestion upset. Besides, methanogenesis was particularly inhibited compared to other three steps and the corresponding enzyme activities decreased by 66%. Furthermore, the polysaccharide contents in EPS increased after exposure to tetracycline, which could inhibit direct connections among microorganism. At last, long-term exposure to tetracycline inhibit anaerobic microbial activities and caused liberation of lactate dehydrogenase. The results would provide novel insights for anaerobic digestion of swine wastewater.

Tonalide facilitates methane production from anaerobic digestion of waste activated sludge

Tonalide (AHTN), a typical polycyclic musk and an emerging pollutant, was found to be enriched in waste activated sludge (WAS). However, the research of its effect on WAS anaerobic digestion was seldom available. This research therefore investigated the effect of AHTN on WAS anaerobic digestion and the underlying mechanism through batch experiments using either real WAS or synthetic wastewaters as the digestion substrates. The results indicated that when the concentration of AHTN increased from 0 to 1000 mg/kg TSS in WAS, the methane production increased linearly from 125.0 ± 2.2 to 162.9 ± 1.6 mL/g VSS, while the AHTN concentration further increased to 2000 mg/kg TSS, the methane production decreased to 146.2 ± 2.1 mL/g VSS. At the same time AHTN can facilitate the utility of volatile fatty acid (VFAs), especially acetate and propionate. It was further found that the degradation efficiency of AHTN in anaerobic digestion was 42.7%. The mechanism investigation demonstrated that AHTN can promote the solubilization, homoacetogenesis, acetogenesis and methanogenesis processes, leading to an increase in methane production. Further analysis revealed that methanogenic archaea mainly belonged to the genera of Methanosaeta and Metheanobacterium, and their relative abundance increased accordingly with the addition of AHTN.

Physiological, biochemical and transcription effects of roxithromycin before and after phototransformation in Chlorella pyrenoidosa

Photodegradation is an important transformation pathway for macrolide antibiotics (MCLs) in aquatic environments, but the ecotoxicity of MCLs after phototransformation has not been reported in detail. This study investigated the effects of roxithromycin (ROX) before and after phototransformation on the growth and physio-biochemical characteristics of Chlorella pyrenoidosa, and its toxicity were explored using transcriptomics analysis. The results showed that 2 mg/L ROX before phototransformation (T0 group) inhibited algae growth with inhibition rates of 53.06%, 54.17%, 47.26%, 31.27%, and 28.38% at 3, 7, 10, 14, and 21 d, respectively, and chlorophyll synthesis was also inhibited. The upregulation of antioxidative enzyme activity levels and the malondialdehyde content indicated that ROX caused oxidative damage to C. pyrenoidosa during 21 d of exposure. After phototransformation for 48 h (T48 group), ROX exhibited no significant impact on the growth and physio-biochemical characteristics of the microalgae. Compared with the control group (without ROX and its phototransformation products), 2010 and 2988 differentially expressed genes were identified in the T0 and T48 treatment groups, respectively. ROX significantly downregulated genes related to porphyrin and chlorophyll metabolism, which resulted in the inhibition of chlorophyll synthesis and algae growth. ROX also significantly downregulated genes of DNA replication, suggesting the increased DNA proliferation risks in algae. After phototransformation, ROX upregulated most of the genes associated with the porphyrin and chlorophyll metabolism pathway, which may be the reason that the chlorophyll content in T48 treatment group showed no significant difference from the control group. Almost all light-harvesting chlorophyll a/b (LHCa/b) gene family members were upregulated in both T0 and T48 treatment groups, which may compensate part of the stress of ROX and its phototransformation products.

Bacterial community composition of internal circulation reactor at different heights for large-scale brewery wastewater treatment

This study analyzed bacterial community structure for large-scale brewery wastewater treatment at different heights in internal circulation (IC) reactor. Proteobacteria, Bacteroidetes and Chloroflexi were dominant bacteria, which accounted for 64.17%, 64.04%, 59.87% and 55.79% in phylum level, respectively. The unidentified bacteria were accounted for a large proportion in genus level, available data showed that Longilinea, Desulfomicrobium, Caldithrix, Geobacter and Syntrophorhabdus were relatively abundant. Organic fermentation, hydrolysis, and acidification were mainly completed at the bottom, and production of hydrogen and methane were completed in the upper and middle part of reactor. Alpha diversity and cluster distance analysis showed the bacterial community could be divided into bottom, middle and upper part of IC reactor. The IC reactor possessed the COD_Cr removal efficiency of 80% - 84.09%, and BOD₅ of 77.50% - 86% for brewery wastewater. This study would provide bacterial analysis references of IC reactor for industrial wastewater treatment in future.

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.