Jun 2019, Volume 13 Issue 3

Cover illustration

  • Discharge of treated sewage and water using only conventional disinfectants may not prevent the release of antibiotic resistant bacteria (ARB) and antibiotics resistance genes (ARGs) in the environment, which have profound consequences to human health. The ultraviolet (UV) technology has been applied in treatment processes, but may not be able to effectively treat ARGs, especially at UV doses used for disinfection. However, the UV technology in conjunction with known oxidan [Detail] ...

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    Xin Yu, Virender K. Sharma, Hui Li
    Christine C. Nguyen, Cody N. Hugie, Molly L. Kile, Tala Navab-Daneshmand

    Heavy metals can act as co-selecting agents and promote antibiotic resistance.

    Most frequent resistances to heavy metals are observed for zinc and cadmium.

    P. aeruginosa and E. coli are commonly resistant to heavy metals and antibiotics.

    Heavy metals proliferate antibiotic resistance through co- and cross-resistance.

    Heavy metal and antibiotic resistances are common near anthropogenic activities.

    Antibiotic resistance in human pathogens can proliferate under selective pressures. Heavy metals in environmental reservoirs may contribute to selecting antibiotic-resistant strains. To determine the associations between heavy metals and antibiotic resistance, a literature review was conducted to systematically collect and categorize evidence for co-occurrence of resistance to heavy metals and antibiotics within human pathogenic bacteria in water, wastewater, and soil. In total, 42 publications adhered to inclusion criteria. Across the reservoirs, zinc and cadmium were the most commonly observed heavy metals associated with resistance to antibiotics. Pseudomonas aeruginosa and Escherichia coli were the most commonly studied bacteria with reported co-occurrence of resistance to several heavy metals and antibiotic classes. As co-selecting agents, prevalence of heavy metals in the environment can proliferate resistance to heavy metals and antibiotics through co-resistance and cross-resistance mechanisms. In comparing different reservoirs, soils and sediments harbor higher heavy metal and antibiotic resistances compared to water environments. Additionally, abiotic factors such as pH can affect the solubility and hence, the availability of heavy metals to bacterial pathogens. Overall, our review demonstrates heavy metals act as co-selecting agents in the proliferation of antibiotic resistance in human pathogens in multiple environmental reservoirs. More studies that include statistical data are needed to further describe the exposure-response relationships between heavy metals and antibiotic resistance in different environmental media. Moreover, integration of culture-based and molecular-based methods in future studies are recommended to better inform our understanding of bacterial co- and cross-resistance mechanisms to heavy metals and antibiotics.

    Xuan Zhu, Chengsong Ye, Yuxin Wang, Lihua Chen, Lin Feng

    • Quantitative global ARGs profile in dialysis water was investigated.

    • Totally 35 ARGs were found in the dialysis treatment train.

    • 29 ARGs (highest) were found in carbon filtration effluent.

    erm and mtrD-02 occurred in the final effluent.

    • The effluent was associated with health risks even after RO treatment.

    Dialysis water is directly related to the safety of hemodialysis patients, thus its quality is generally ensured by a stepwise water purification cascade. To study the effect of water treatment on the presence of antibiotic resistance genes (ARGs) in dialysis water, this study used propidium monoazide (PMA) in conjunction with high throughput quantitative PCR to analyze the diversity and abundance of ARGs found in viable bacteria from water having undergone various water treatment processes. The results indicated the presence of 35 ARGs in the effluents from the different water treatment steps. Twenty-nine ARGs were found in viable bacteria from the effluent following carbon filtration, the highest among all of the treatment processes, and at 6.96 Log (copies/L) the absolute abundance of the cphA gene was the highest. Two resistance genes, erm (36) and mtrD-02, which belong to the resistance categories macrolides-lincosamides-streptogramin B (MLSB) and other/efflux pump, respectively, were detected in the effluent following reverse osmosis treatment. Both of these genes have demonstrated the potential for horizontal gene transfer. These results indicated that the treated effluent from reverse osmosis, the final treatment step in dialysis-water production, was associated with potential health risks.

    Gastón Azziz, Matías Giménez, Héctor Romero, Patricia M. Valdespino-Castillo, Luisa I. Falcón, Lucas A. M. Ruberto, Walter P. Mac Cormack, Silvia Batista

    • Beta-lactamase genes were found in all samples from distant places in Antarctica.

    • Class C beta-lactamase coding genes were the most frequently found.

    • Diversity of sequences exceeds that of the beta-lactamases from clinical environment.

    Analysis of environmental samples for bacterial antibiotic resistance genes may have different objectives and analysis strategies. In some cases, the purpose was to study diversity and evolution of genes that could be grouped within a mechanism of antibiotic resistance. Different protocols have been designed for detection and confirmation that a functional gene was found. In this study, we present a sequence-based screening of candidate genes encoding beta-lactamases in 14 metagenomes of Antarctic microbial mats. The samples were obtained from different sites, representing diverse biogeographic regions of maritime and continental Antarctica. A protocol was designed based on generation of Hidden Markov Models from the four beta-lactamase classes by Ambler classification, using sequences from the Comprehensive Antibiotic Resistance Database (CARD). The models were used as queries for metagenome analysis and recovered contigs were subsequently annotated using RAST. According to our analysis, 14 metagenomes analyzed contain A, B and C beta-lactamase genes. Class D genes, however, were identified in 11 metagenomes. The most abundant was class C (46.8%), followed by classes B (35.5%), A (14.2%) and D (3.5%). A considerable number of sequences formed clusters which included, in some cases, contigs from different metagenomes. These assemblies are clearly separated from reference clusters, previously identified using CARD beta-lactamase sequences. While bacterial antibiotic resistance is a major challenge of public health worldwide, our results suggest that environmental diversity of beta-lactamase genes is higher than that currently reported, although this should be complemented with gene function analysis.

    Mengli Wang, Ruying Li, Qing Zhao

    Sludge digestion is critical to control the spread of ARGs from wastewater to soil.

    Fate of ARGs in three pretreatment-AD processes was investigated.

    UP was more efficient for ARGs removal than AP and THP in pretreatment-AD process.

    The total ARGs concentration showed significant correlation with 16S rRNA gene.

    The bacteria carrying ARGs could be mainly affiliated with Proteobacteria.

    Sewage sludge in the wastewater treatment plants contains considerable amount of antibiotic resistance genes (ARGs). A few studies have reported that anaerobic digestion (AD) could successfully remove some ARGs from sewage sludge, but information on the fate of ARGs in sludge pretreatment-AD process is still very limited. In this study, three sludge pretreatment methods, including alkaline, thermal hydrolysis and ultrasonic pretreatments, were compared to investigate the distribution and removal of ARGs in the sludge pretreatment-AD process. Results showed that the ARGs removal efficiency of AD itself was approximately 50.77%, and if these three sludge pretreatments were applied, the total ARGs removal efficiency of the whole pretreatment-AD process could be improved up to 52.50%–75.07%. The ultrasonic pretreatment was more efficient than alkaline and thermal hydrolysis pretreatments. Although thermal hydrolysis reduced ARGs obviously, the total ARGs rebounded considerably after inoculation and were only removed slightly in the subsequent AD process. Furthermore, it was found that the total ARGs concentration significantly correlated with the amount of 16S rRNA gene during the pretreatment and AD processes, and the bacteria carrying ARGs could be mainly affiliated with Proteobacteria.

    G. S. Muthu Iswarya, B. Nirkayani, A. Kavithakani, V. C. Padmanaban

    Linear, interactive and quadratic effects of process parameters were studied.

    Degradation of Ofloxacin (Ofx) was related with G value of irradiation process.

    The synergistic effect of H2O2 on lower dose of g-irradiation was established.

    The process follows pseudo first order with dose constant (d = 0.232 kGy1).

    The impact of human activities in the past few decades has paved the way for the release of pollutants due to the improper effluent treatment. Recent studies revealed that, Ofloxacin, an antibiotic as one of the major pollutant affecting surface water and ground water. In this study, the radiolytic potential of Ofloxacin was investigated. The effects of pH, dose and concentration of Ofloxacin were analyzed using One Factor At a Time (OFAT) and the interactive effects between the parameters were studied using Face Centered Central Composite Design. The statistically optimised developed model shows 30% degradation at initial antibiotic concentration of 1mM at pH 3.0 and at 2 kGy dose of gamma ray. The process efficiency was evaluated in terms of G value and its correlation with the concentration of antibiotic was also established. The process of degradation was augmented by the addition of H2O2 (1.5 mM). The reaction kinetics for the process was evaluated, the dose rate constant and the rate of degradation for the augmented process was found to be 0.232 kGy-1 and 0.232 mM/kGy, respectively. The degraded metabolites of the radiolytic degradation of Ofloxacin were analyzed through change in pH, reduction in TOC and GC-MS spectrum.

    Bao Yu, Guodi Zheng, Xuedong Wang, Min Wang, Tongbin Chen

    TCS and TCC can be biodegraded during sewage sludge composting.

    Ventilation significantly accelerated the biodegradation of TCS and TCC in sludge.

    Composting can reduce the environmental risk of TCS and TCC in sewage sludge.

    Triclosan (TCS) and triclocarban (TCC) are widely used in home and personal care products as antimicrobial agents. After these products are used, TCS and TCC enter the terrestrial environment and pose a great risk to humans and animals. In this research, the biodegradation of TCS and TCC was investigated during sewage sludge composting with ventilation rates of 108, 92, and 79 m3/min. TCS and TCC were mainly biodegraded in the mesophilic and thermophilic phases, and the biodegradation rates improved with an increase in ventilation. After sewage sludge was composted for 16 days with forced ventilation (108 m3/min), the concentration of TCS decreased from 497.4 to 214.5 μg/kg, and the concentration of TCC decreased from 823.2 to 172.7 μg/kg. The biodegradation rates of TCS and TCC were 65.2% and 83.1%, respectively. However, after the sewage sludge was stacked for 16 days, the biodegradation rates of TCS and TCC were only 17.0% and 18.2%, respectively. The environmental risks of TCS and TCC in the sewage sludge piles significantly decreased after composting. In the sludge pile with a ventilation rate of 108 m3/min, the RQ values of TCS and TCC decreased from 8.29 and 20.58 to 3.58 and 4.32 after composting for 16 days, respectively. There is still a high risk if the sludge compost is directly used as a culture substrate. Nevertheless, the environmental risk could be decreased distinctly if a reasonable quantity of sludge compost is applied to land to ensure an RQ of<1 for TCS and TCC.

    Monika Nowrotek, Łukasz Jałowiecki, Monika Harnisz, Grażyna Anna Płaza

    State of the art of culturomics and metagenomics to study resistome was presented.

    The combination of culturomics and metagenomics approaches was proposed.

    The research directions of antibiotic resistance study has been suggested.

    Pharmaceutical residues, mainly antibiotics, have been called “emerging contaminants” in the environment because of their increasing frequency of detection in aquatic and terrestrial systems and their sublethal ecological effects. Most of them are undiscovered. Both human and veterinary pharmaceuticals, including antibiotics, are introduced into the environment via many different routes, including discharges from municipal wastewater treatment plants and land application of animal manure and biosolids to fertilize croplands. To gain a comprehensive understanding of the widespread problem of antibiotic resistance, modern and scientific approaches have been developed to gain knowledge of the entire antibiotic-resistant microbiota of various ecosystems, which is called the resistome. In this review, two omics methods, i.e. culturomics, a new approach, and metagenomics, used to study antibiotic resistance in environmental samples, are described. Moreover, we discuss how both omics methods have become core scientific tools to characterize microbiomes or resistomes, study natural communities and discover new microbes and new antibiotic resistance genes from environments. The combination of the method for get better outcome of both culturomics and metagenomics will significantly advance our understanding of the role of microbes and their specific properties in the environment.

    Yangyang Yu, Xiaolin Zhu, Guanlan Wu, Chengzhi Wang, Xing Yuan

    The concentrations of four types of antibiotics in the Yitong River were detected.

    The concentration of total coliforms in summer was higher than that in spring.

    There was a seasonal difference in antibiotic resistance of E. coli.

    The E. coli in the Yitong River was found to have multiple antibiotic resistance.

    The Yitong River is one of the largest secondary tributaries of the Songhua River. The area where the Yitong River flows is densely populated and contains the livestock and poultry breeding areas of north-east China. These areas introduce a high risk of antibiotic contamination. In this study, the concentrations of four types of typical antibiotics including quinolones, tetracyclines, sulfonamides, and trimethoprim were determined by solid phase extraction-high performance liquid chromatography. The antibiotic resistance of Escherichia coli caused by antibiotic pollution was investigated. The concentration of total coliforms in the Yitong River was detected by the plate counting method. The antibiotic resistance of E. coli to quinolones, tetracyclines, sulfonamides, and trimethoprim was analyzed by the Kirby-Bauer method. The results showed that the concentration of total coliforms in the summer was higher than that in the spring. There was a seasonal difference in the resistance rate of E. coli to antibiotics except trimethoprim. The antibiotic resistance to fluoroquinolones was relatively low. The resistance rate to tetracyclines was higher during the summer. Moreover, resistance to several antibiotics was observed in all sections. This study provides basic data for research on pollution characteristics and prevention of antibiotic exposure in rivers.

    Menglu Zhang, Sheng Chen, Xin Yu, Peter Vikesland, Amy Pruden

    Extracellular DNA structure damaged by chlorination was characterized.

    Integrity of extracellular ARG genetic information after chlorination was determined.

    Typical chlorine doses will likely effectively diminish extracellular DNA and ARGs.

    Plasmid DNA/ARGs were less readily broken down than genomic DNA.

    The Bioanalyzer methodology effectively documented damage incurred to DNA.

    There is a need to improve understanding of the effect of chlorine disinfection on antibiotic resistance genes (ARGs) in order to advance relevant drinking water, wastewater, and reuse treatments. However, few studies have explicitly assessed the physical effects on the DNA. Here we examined the effects of free chlorine (1–20 mg Cl2/L) on extracellular genomic, plasmid DNA and select ARGs. Chlorination was found to decrease the fluorometric signal of extracellular genomic and plasmid DNA (ranging from 0.005 to 0.05 mg/mL) by 70%, relative to a no-chlorine control. Resulting DNA was further subject to a fragment analysis using a Bioanalyzer, indicating that chlorination resulted in fragmentation. Moreover, chlorine also effectively deactivated both chromosomal- and plasmid-borne ARGs, mecA and tetA, respectively. For concentrations >2 mg Cl2//L × 30 min, chlorine efficiently reduced the qPCR signal when the initial concentration of ARGs was 105 copies/mL or less. Notably, genomic DNA and mecA gene signals were more readily reduced by chlorine than the plasmid-borne tetA gene (by ~2 fold). Based on the results of qPCR with short (~200 bps) and long amplicons (~1200 bps), chlorination could destroy the integrity of ARGs, which likely reduces the possibility of natural transformation. Overall, our findings strongly illustrate that chlorination could be an effective method for inactivating extracellular chromosomal- and plasmid-borne DNA and ARGs.

    Virender K. Sharma, Xin Yu, Thomas J. McDonald, Chetan Jinadatha, Dionysios D. Dionysiou, Mingbao Feng

    Antibiotic-resistant bacteria and antibiotic resistance genes are in water bodies.

    UV/chlorination method is better to remove ARGs than UV or chlorination alone.

    Research on UV/hydrogen peroxide to eliminate ARGs is forthcoming.

    UV-based photocatalytic processes are effective to degrade ARGs.

    Antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) have been recognized as one of the biggest public health issues of the 21st century. Both ARB and ARGs have been determined in water after treatment with conventional disinfectants. Ultraviolet (UV) technology has been seen growth in application to disinfect the water. However, UV method alone is not adequate to degrade ARGs in water. Researchers are investigating the combination of UV with other oxidants (chlorine, hydrogen peroxide (H2O2), peroxymonosulfate (PMS), and photocatalysts) to harness the high reactivity of produced reactive species (Clž·, ClOž·ž, Clž2·ž, žž·OH, and SOž4ž·€) in such processes with constituents of cell (e.g., deoxyribonucleic acid (DNA) and its components) in order to increase the degradation efficiency of ARGs. This paper briefly reviews the current status of different UV-based treatments (UV/chlorination, UV/H2O2, UV/PMS, and UV-photocatalysis) to degrade ARGs and to control horizontal gene transfer (HGT) in water. The review also provides discussion on the mechanism of degradation of ARGs and application of q-PCR and gel electrophoresis to obtain insights of the fate of ARGs during UV-based treatment processes.

    Qiaowen Tan, Weiying Li, Junpeng Zhang, Wei Zhou, Jiping Chen, Yue Li, Jie Ma

    Reviewed the change of ARGs and ARB in full-scale urban drinking water systems.

    Conventional processes are more promising than BAC process in ARGs removal.

    Mechanisms of ARGs enrichment and spread in BAC filter and DWDSs are discussed.

    Raise the need of future research on ARGs and ARB change in building plumbing systems.

    Antibiotic resistance in aquatic environment has become an important pollution problem worldwide. In recent years, much attention was paid to antibiotic resistance in urban drinking water systems due to its close relationship with the biosafety of drinking water. This review was focused on the mechanisms of antibiotic resistance, as well as the presence, dissemination and removal of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) in the urban drinking water system. First, the presence of ARB and ARGs in the drinking water source was discussed. The variation of concentration of ARGs and ARB during coagulation, sedimentation and filtration process were provided subsequently, in which filtration was proved to be a promising technology to remove ARGs. However, biological activated carbon (BAC) process and drinking water distribution systems (DWDSs) could be incubators which promote the antibiotic resistance, due to the enrichment of ARGs and ARB in the biofilms attached to the active carbon and pipe wall. Besides, as for disinfection process, mechanisms of the inactivation of ARB and the promotion of conjugative transfer of ARGs under chlorine, ozone and UV disinfection were described in detail. Here we provide some theoretical support for future researches which aim at antibiotic resistance controlling in drinking water.

    Chengjun Pu, Xiaoyan Gong, Ying Sun

    pRKZ3 is a non-conjugative IncQ plasmid, while pKANJ7 is a conjugative IncX plasmid.

    The optimal mating time of pKANJ7 varied under different conditions.

    Both of the two transferable ARPs had little impact on the growth of their hosts.

    A relatively high level of fitness cost was observed for pKANJ7.

    The fitness cost of ARPs depended on their hosts.

    Plasmid-mediated antibiotic resistance genes (ARGs) have recently become a more prominent concern in the global environment. However, the prevalence of aminoglycoside resistance plasmids in the livestock industry is under reported. In this study, two transferable aminoglycoside resistance plasmids, pRKZ3 and pKANJ7, isolated from pig and chicken manure, were characterized. Results showed that pRKZ3 (8236 bp) is a non-conjugative IncQ plasmid and contains genes encoding for plasmid replication and stabilization (repA, repB and repC), mobilization (mob), and antibiotic resistance (arr-3 and aacA). pKANJ7 (30142 bp) is a conjugative IncX plasmid which codes for a type IV secretion system (T4SS). Conjugative transfer experiments showed that the optimal mating time of pKANJ7 was 8 h under the starvation condition, but the number of tranconjugants increased with time under the nutrient condition. Statistical analysis indicated that the two plasmids had little impact on the growth of their hosts, but a relatively high level of fitness cost due to pKANJ7 was observed. We also found that the fitness cost of plasmids depended on their hosts. Compared with pKANJ7, the relative fitness cost index of pRKZ3 varied within a narrow range during the 10 days of competition. The low level of fitness cost of pRKZ3 might contribute to the persistence of the plasmid in the environment. Our study provides new information for understanding the characterizations of antibiotic resistance plasmids (ARPs) in manure sources and helps to clarify the transfer and persistence of ARPs in the environment following the application of manure.

    Weihua Wang, Wanfeng Zhang, Hong Liang, Dawen Gao

    • The concentration distributions and removals of 12 antibiotics were investigated.

    • Macrolides and fluoroquinolones presented relatively higher concentrations.

    • The removal of antibiotics did not differ among the different treatment processes.

    As a new pollutant, antibiotics in the environment and their removal in wastewater treatment plants (WWTPs) have gained considerable attention. However, few studies investigated antibiotics in the north-eastern areas of China. By employing high-performance liquid chromatography-tandem mass spectrometry, the concentration distribution and removal of 12 antibiotics belonging to four types (cephalosporins, sulfonamides, fluoroquinolones, and macrolides) were investigated in influents and effluents as well as the removal efficiencies of four typical treatment processes in 18 wastewater treatment plants of Harbin City, north-eastern China. Macrolides and fluoroquinolones presented relatively higher detection concentrations and rates both in the water and in the sludge phases. Sulfonamides and cephalosporins displayed higher detection concentrations in the water phase. The representative antibiotics in influents and effluents included ROX, NOR, OFL, SMX, AZI, and CLA, with detection rates above 95%. The detection rates of the other six antibiotics exceeded 30%. In the sludge, the other five antibiotics, except SMX, presented relatively higher detection concentrations and rates, with detection rates greater than 95%. For cephalosporin, the detection rates of CTX and CFM in the sludge were 0. The removal efficiencies for the 12 antibiotics differed greatly, with average values ranging from 30% to 80% and the highest values for cephalosporin. The removal of antibiotics did not differ significantly among the four different treatment processes.

    Zhipeng Yang, Anxing Lai, Hangyu Chen, Youxiang Yan, Ye Yang, Weiwei Zhang, Lei Wang

    The discharge characteristics during the degradation of MNZ by DBD were investigated.

    Increasing the discharge frequency can promote the degradation of MNZ.

    MNZ removal reaches 99.1% at the initial concentration of 40 ppm within 120 min.

    Coexisting organic matter inhibits the degradation of MNZ.

    The energy efficiency of DBD for MNZ removal is higher than other technologies.

    Degradation of metronidazole (MNZ) which is a representative and stable antibiotic by dielectric barrier discharge (DBD) in an aqueous solution has been studied. Effects of initial MNZ concentration, solution pH and coexisting organics on the degradation were investigated. The results illustrated that increasing the input power and the discharge frequency can improve the removal of MNZ. At low initial concentration, the removal of MNZ can reach up to 99.1%. Acidic and neutral conditions are more favorable for MNZ removal than alkaline condition in the early stage of degradation. However, the difference in MNZ removal between those in acidic or neutral media and that in alkaline one could be neglected with prolonging of the treatment time. Therefore, this method can be applied to MNZ degradation with a wide pH range. Coexisting organic matter in water can attenuate the removal to some extent. This study could provide valuable references for the degradation of nitroimidazole antibiotics by DBD.

    Lian Yang, Qinxue Wen, Zhiqiang Chen, Ran Duan, Pan Yang

    The distributions of ARGs were monitored in a WWTP in Harbin during six months.

    CASS had the best removal efficacy of ARGs compared to other processes in the WWTP.

    UV disinfection could effectively control the HGT.

    AGAC significantly remove ARGs and organics due to its high absorption capacity.

    Combination of ozone and AGAC significantly improve removal of ARGs and organics.

    Antibiotic resistance genes (ARGs) pose a serious threat to public health. Wastewater treatment plants (WWTPs) are essential for controlling the release of ARGs into the environment. This study investigated ARG distribution at every step in the treatment process of a municipal WWTP located in Harbin for six consecutive months. Changes in ARG distribution involved in two advanced secondary effluent treatment processes, ozonation and granular activated carbon (GAC) adsorption, were analyzed. Biological treatment resulted in the highest ARG removal (0.76–1.94 log reduction), followed by ultraviolet (UV) disinfection (less than 0.5-log reduction). Primary treatment could not significantly remove ARGs. ARG removal efficiency increased with an increase in the ozone dose below 40 mg/L. However, amorphous GAC (AGAC) adsorption with a hydraulic retention time (HRT) of 1 h showed better removal of ARGs, total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) than ozonation at a 60 mg/L dose. UV treatment could efficiently reduce the relative ARG abundance, despite presenting the lowest efficiency for the reduction of absolute ARG abundance compared with GAC and ozone treatments. The combination of ozone and AGAC can significantly improve the removal of ARGs, TOC, TN and TP. These results indicate that a treatment including biological processing, ozonation, and AGAC adsorption is a promising strategy for removing ARGs and refractory organic substances from sewage.

    Jie Liao, Chaoxiang Liu, Lin Liu, Jie Li, Hongyong Fan, Jiaqi Ye, Zhichao Zeng

    Longer HRT can enhance degradation rate of sulfamethoxazole in granular reactor.

    Longer HRT can reduce accumulated concentrations of TCs and QNs in sludge.

    Longer HRT may have increased relative abundances of ARGs in aerobic granules.

    The behavior of antibiotics and the corresponding resistance genes in aerobic granular reactors for treating biogas slurry under different hydraulic retention times (10.7 h, R1; 8 h, R2) was investigated in this study. The results indicated that the hydraulic retention time could affect the effluent concentrations and removal efficiencies of sulfonamides. The average removal rates of tetracyclines, fluoroquinolones, and sulfonamides were 63%, 46%, and 90% in R1, and 62%, 46%, and 86% in R2, respectively. Although the removal efficiencies of tetracyclines and fluoroquinolones were similar in both reactors, the respective accumulated concentrations of tetracyclines and fluoroquinolones in R1 were 7.00 and 11.15 µg/g SS, which were lower than those in R2 (8.92 and 13.37 µg/g SS, respectively). The difference in the relative abundance of target antibiotic resistance genes between both reactors was not significant, yet the average relative abundances of all target resistance genes in R1 were higher than those in R2 after 45 days of operation. The results of this study suggested that a longer hydraulic retention time could enhance the antibiotic removal ability of aerobic granular sludge, yet it may also increase the risk of surplus sludge utilization from a resistance genes point of view.