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Błaszczyk W, Siatecka A, Tlustoš P, Oleszczuk P. Occurrence and dissipation mechanisms of organic contaminants during sewage sludge anaerobic digestion: A critical review. THE SCIENCE OF THE TOTAL ENVIRONMENT 2024; 945:173517. [PMID: 38821290 DOI: 10.1016/j.scitotenv.2024.173517] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/12/2024] [Revised: 05/23/2024] [Accepted: 05/24/2024] [Indexed: 06/02/2024]
Abstract
Sewage sludge, a complex mixture of contaminants and pathogenic agents, necessitates treatment or stabilization like anaerobic digestion (AD) before safe disposal. AD-derived products (solid digestate and liquid fraction) can be used as fertilizers. During AD, biogas is also produced, and used for energy purposes. All these fractions can be contaminated with various compounds, whose amount depends on the feedstocks used in AD (and their mutual proportions). This paper reviews studies on the distribution of organic contaminants across AD fractions (solid digestate, liquid fraction, and biogas), delving into the mechanisms behind contaminant dissipation and proposing future research directions. AD proves to be a relatively effective method for removing polychlorinated biphenyls, polycyclic aromatic hydrocarbons, pharmaceuticals, antibiotic resistance genes and hydrocarbons. Contaminants are predominantly removed through biodegradation, but many compounds, especially hydrophobic (e.g. per- and polyfluoroalkyl substances), are also sorbed onto digestate particles. The process of sorption is suggested to reduce the bioavailability of contaminants. As a result of sorption, contaminants accumulate in the largest amount in the solid digestate, whereas in smaller amounts in the other AD products. Polar pharmaceuticals (e.g. metformin) are particularly leached, while volatile methylsiloxanes and polycyclic aromatic hydrocarbons, characterized by a high Henry's law constant, are volatilized into the biogas. The removal of compounds can be affected by AD operational parameters, the type of sludge, physicochemical properties of contaminants, and the sludge pretreatment used.
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Affiliation(s)
- Wiktoria Błaszczyk
- Department of Chemistry, Faculty of Food Science and Biotechnology, University of Life Sciences, 15 Akademicka Street, 20-950 Lublin, Poland
| | - Anna Siatecka
- Department of Chemistry, Faculty of Food Science and Biotechnology, University of Life Sciences, 15 Akademicka Street, 20-950 Lublin, Poland
| | - Pavel Tlustoš
- Department of Agro-Environmental Chemistry and Plant Nutrition, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague, 129 Kamýcká Street, Praha 6 - Suchdol 165 00, Czech Republic
| | - Patryk Oleszczuk
- Department of Radiochemistry and Environmental Chemistry, Institute of Chemical Sciences, Faculty of Chemistry, Maria Curie-Sklodowska University, 3 Maria Curie-Sklodowska Square, 20-031 Lublin, Poland.
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Porras-Socias P, Tomasino MP, Fernandes JP, De Menezes AB, Fernández B, Collins G, Alves MJ, Castro R, Gomes CR, Almeida CMR, Mucha AP. Removal of metals and emergent contaminants from liquid digestates in constructed wetlands for agricultural reuse. Front Microbiol 2024; 15:1388895. [PMID: 38903785 PMCID: PMC11187104 DOI: 10.3389/fmicb.2024.1388895] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2024] [Accepted: 05/17/2024] [Indexed: 06/22/2024] Open
Abstract
Given the increasing pressure on water bodies, it is imperative to explore sustainable methodologies for wastewater treatment and reuse. The simultaneous presence of multiples contaminants in complex wastewater, such as the liquid effluents from biogas plants, can compromise biological treatment effectiveness for reclaiming water. Vertical subsurface flow constructed wetlands were established as low-cost decentralized wastewater treatment technologies to treat the liquid fraction of digestate from municipal organic waste with metals, antibiotics, and antibiotic resistance genes, to allow its reuse in irrigation. Twelve lab-scale planted constructed wetlands were assembled with gravel, light expanded clay aggregate and sand, testing four different treating conditions (liquid digestate spiked with oxytetracycline, sulfadiazine, or ofloxacin, at 100 μg/ L, or without dosing) during 3 months. Physicochemical parameters (pH, chemical oxygen demand (COD), nutrients, metals, and antibiotics), the microbial communities dynamics (through 16S high-throughput sequencing) and antibiotic resistance genes removal (qPCR) were monitored in influents and effluents. Systems removed 85.8%-96.9% of organic matter (as COD), over 98.1% of ammonium and phosphate ions, and 69.3%-99.4% of nitrate and nitrite ions, with no significant differences between the presence or absence of antibiotics. Removal of Fe, Mn, Zn, Cu, Pb and Cr exceeded 82% in all treatment cycles. The treatment also removed oxytetracycline, sulfadiazine and ofloxacin over 99%, and decreased intl1, tetA, tetW, sul1 and qnrS gene copies. Nonetheless, after 3 months of ofloxacin dosing, qnrS gene started being detected. Removal processes relied on high HRT (14 days) and various mechanisms including sorption, biodegradation, and precipitation. Microbial community diversity in liquid digestate changed significantly after treatment in constructed wetlands with a decrease in the initial Firmicutes dominance, but with no clear effect of antibiotics on the microbial community structure. Removals above 85% and 94% were observed for Streptococcus and Clostridium, respectively. Results suggest that vertical subsurface flow constructed wetlands were a suitable technology for treating the liquid digestate to reuse it in irrigation agricultural systems, contributing to the circular bioeconomy concept. However, a more profound understanding of effective wastewater treatment strategies is needed to avoid antibiotic resistance genes dissemination.
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Affiliation(s)
- Pau Porras-Socias
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
- Chemistry and Biochemistry Department, Faculty of Sciences, University of Porto, Porto, Portugal
- Microbiology, School of Biological and Chemical Sciences and Ryan Institute, University of Galway, Galway, Ireland
- Sustainability in Biosystems Programme, IRTA, Institute of Agrifood Research and Technology, Caldes de Montbui, Spain
| | - Maria Paola Tomasino
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
| | - Joana P. Fernandes
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
| | - Alexandre B. De Menezes
- Microbiology, School of Biological and Chemical Sciences and Ryan Institute, University of Galway, Galway, Ireland
| | - Belén Fernández
- Sustainability in Biosystems Programme, IRTA, Institute of Agrifood Research and Technology, Caldes de Montbui, Spain
| | - Gavin Collins
- Microbiology, School of Biological and Chemical Sciences and Ryan Institute, University of Galway, Galway, Ireland
| | - Maria João Alves
- TratoLixo—Tratamento de Resíduos Sólidos, E.I.M. S.A., São Domingos de Rana, Portugal
| | - Ricardo Castro
- TratoLixo—Tratamento de Resíduos Sólidos, E.I.M. S.A., São Domingos de Rana, Portugal
| | - Carlos R. Gomes
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
- Chemistry and Biochemistry Department, Faculty of Sciences, University of Porto, Porto, Portugal
| | - C. Marisa R. Almeida
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
- Chemistry and Biochemistry Department, Faculty of Sciences, University of Porto, Porto, Portugal
| | - Ana Paula Mucha
- CIIMAR, Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Matosinhos, Portugal
- Biology Department, Faculty of Sciences, University of Porto, Porto, Portugal
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Wolak I, Bajkacz S, Harnisz M, Stando K, Męcik M, Korzeniewska E. Digestate from Agricultural Biogas Plants as a Reservoir of Antimicrobials and Antibiotic Resistance Genes-Implications for the Environment. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH 2023; 20:2672. [PMID: 36768038 PMCID: PMC9915926 DOI: 10.3390/ijerph20032672] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/22/2022] [Revised: 01/30/2023] [Accepted: 01/31/2023] [Indexed: 06/18/2023]
Abstract
Antimicrobials and antibiotic resistance genes (ARGs) in substrates processed during anaerobic digestion in agricultural biogas plants (BPs) can reach the digestate (D), which is used as fertilizer. Antimicrobials and ARGs can be transferred to agricultural land, which increases their concentrations in the environment. The concentrations of 13 antibiotics in digestate samples from biogas plants (BPs) were investigated in this study. The abundance of ARGs encoding resistance to beta-lactams, tetracyclines, sulfonamides, fluoroquinolones, macrolide-lincosamide-streptogramin antibiotics, and the integrase genes were determined in the analyzed samples. The presence of cadmium, lead, nickel, chromium, zinc, and mercury was also examined. Antimicrobials were not eliminated during anaerobic digestion. Their concentrations differed in digestates obtained from different substrates and in liquid and solid fractions (ranging from 62.8 ng/g clarithromycin in the solid fraction of sewage sludge digestate to 1555.9 ng/L doxycycline in the liquid fraction of cattle manure digestate). Digestates obtained from plant-based substrates were characterized by high concentrations of ARGs (ranging from 5.73 × 102 copies/gDcfxA to 2.98 × 109 copies/gDsul1). The samples also contained mercury (0.5 mg/kg dry mass (dm)) and zinc (830 mg/kg dm). The results confirmed that digestate is a reservoir of ARGs (5.73 × 102 to 8.89 × 1010 copies/gD) and heavy metals (HMs). In addition, high concentrations of integrase genes (105 to 107 copies/gD) in the samples indicate that mobile genetic elements may be involved in the spread of antibiotic resistance. The study suggested that the risk of soil contamination with antibiotics, HMs, and ARGs is high in farms where digestate is used as fertilizer.
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Affiliation(s)
- Izabela Wolak
- Department of Water Protection Engineering and Environmental Microbiology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Prawocheńskiego 1, 10-720 Olsztyn, Poland
| | - Sylwia Bajkacz
- Department of Environmental Biotechnology, Faculty of Energy and Environmental Engineering, Silesian University of Technology, Akademicka 2, 44-100 Gliwice, Poland
| | - Monika Harnisz
- Department of Water Protection Engineering and Environmental Microbiology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Prawocheńskiego 1, 10-720 Olsztyn, Poland
| | - Klaudia Stando
- Department of Environmental Biotechnology, Faculty of Energy and Environmental Engineering, Silesian University of Technology, Akademicka 2, 44-100 Gliwice, Poland
| | - Magdalena Męcik
- Department of Water Protection Engineering and Environmental Microbiology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Prawocheńskiego 1, 10-720 Olsztyn, Poland
| | - Ewa Korzeniewska
- Department of Water Protection Engineering and Environmental Microbiology, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, Prawocheńskiego 1, 10-720 Olsztyn, Poland
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Gaballah MS, Guo J, Hassanein A, Sobhi M, Zheng Y, Philbert M, Li B, Sun H, Dong R. Removal performance and inhibitory effects of combined tetracycline, oxytetracycline, sulfadiazine, and norfloxacin on anaerobic digestion process treating swine manure. THE SCIENCE OF THE TOTAL ENVIRONMENT 2023; 857:159536. [PMID: 36280067 DOI: 10.1016/j.scitotenv.2022.159536] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2022] [Revised: 10/12/2022] [Accepted: 10/13/2022] [Indexed: 06/16/2023]
Abstract
Combined veterinary antibiotics (CVAs) belonging to different antibiotics classes could cause exacerbated impacts on the anaerobic digestion (AD) process of swine manure. Four different antibiotics "two tetracyclines: tetracycline (TC) and oxytetracycline (OTC), one fluoroquinolones: norfloxacin (Norf), and one sulfonamides: sulfadiazine (SDZ)" were combined to evaluate their removal performances and its inhibition effects on AD. Results indicated that CVAs removal decreased from 84.3 to 63.7 %, with an increase in the initial concentration from 12.5 to 50 mg L-1, where the removal of CVAs occurring in the order OTC > TC > Norf > SDZ. An average of 9.5, 7.5, 9.5, and 32.1 % of the spiked TC, OTC, SDZ, and Norf were remained in the sludge, respectively. With 50 mg L-1 of CVAs, a competitive adsorption phenomenon was found to have a notable impact on biodegradation microorganisms' activity leading a 73.1 % decrease in CH4 production. CVAs caused a temporal inhibition to the acidogenic activity followed by partial inhibition to methanogenic by 66.8 %, and IC50 was 38.5 mg L-1. Moreover, CVAs resulted in acetate accumulation, while 26 % and 48 % lower in TS and COD removal, respectively, were observed. A significant reduction in the relative abundance of bacteria and archaeal genera was also mentioned. The findings of this research would provide a more in-depth understanding of AD's performance in treating swine manure contaminated with combined antibiotics.
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Affiliation(s)
- Mohamed S Gaballah
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China; National Institute of Oceanography and Fisheries, Marine Environment Division, NIOF, Egypt
| | - Jianbin Guo
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China.
| | - Amro Hassanein
- Department of Environmental Science and Technology, University of Maryland, College Park, MD 20742, USA
| | - Mostafa Sobhi
- Agricultural and Bio-systems Engineering Department, Faculty of Agriculture, Alexandria University, Alexandria, Egypt
| | - Yonghui Zheng
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China
| | - Mperejekumana Philbert
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China
| | - Bowen Li
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China
| | - Hui Sun
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China
| | - Renjie Dong
- College of Engineering (Key Laboratory for Clean Renewable Energy Utilization Technology, Ministry of Agriculture), China Agricultural University, Beijing 100083, PR China
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Czatzkowska M, Wolak I, Harnisz M, Korzeniewska E. Impact of Anthropogenic Activities on the Dissemination of ARGs in the Environment-A Review. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH AND PUBLIC HEALTH 2022; 19:ijerph191912853. [PMID: 36232152 PMCID: PMC9564893 DOI: 10.3390/ijerph191912853] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/25/2022] [Revised: 09/28/2022] [Accepted: 10/03/2022] [Indexed: 05/07/2023]
Abstract
Over the past few decades, due to the excessive consumption of drugs in human and veterinary medicine, the antimicrobial resistance (AR) of microorganisms has risen considerably across the world, and this trend is predicted to intensify. Many worrying research results indicate the occurrence of pools of AR, both directly related to human activity and environmental factors. The increase of AR in the natural environment is mainly associated with the anthropogenic activity. The dissemination of AR is significantly stimulated by the operation of municipal facilities, such as wastewater treatment plants (WWTPs) or landfills, as well as biogas plants, agriculture and farming practices, including animal production and land application of manure. These activities entail a risk to public health by spreading bacteria resistant to antimicrobial products (ARB) and antibiotic resistance genes (ARGs). Furthermore, subinhibitory concentrations of antimicrobial substances additionally predispose microbial consortia and resistomes to changes in particular environments that are permeated by these micropollutants. The current state of knowledge on the fate of ARGs, their dissemination and the complexity of the AR phenomenon in relation to anthropogenic activity is inadequate. This review summarizes the state-of-the-art knowledge on AR in the environment, in particular focusing on AR spread in an anthropogenically altered environment and related environmental consequences.
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Effects of Ciprofloxacin Alone or in Mixture with Sulfamethoxazole on the Efficiency of Anaerobic Digestion and Its Microbial Community. Antibiotics (Basel) 2022; 11:antibiotics11081111. [PMID: 36009981 PMCID: PMC9404932 DOI: 10.3390/antibiotics11081111] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2022] [Revised: 08/08/2022] [Accepted: 08/12/2022] [Indexed: 01/04/2023] Open
Abstract
Some livestock farms rely on anaerobic digestion (AD) technology for manure disposal, thus obtaining energy (biogas) and fertilizer (digestate). Mixtures of antibiotics used for animal health often occur in organic waste and their possible synergistic/antagonistic effects on microorganisms involved in AD are still poorly studied. This work focuses on the effects of adding ciprofloxacin, alone (5 mg L−1) and in combination with sulfamethoxazole (2.5–5–10 mg L−1), on AD efficiency and microbial community structure. The experiment consisted of 90-day cattle manure batch tests and antibiotic removal percentages were assessed. Adding antibiotics always promoted CH4 and H2 production compared to untreated controls; however, CH4 production was lowered with the highest ciprofloxacin (CIP) concentrations. The overall results show antibiotic degradation caused by acidogenic Bacteria, and CH4 was mainly produced through the hydrogenotrophic-pathway by methanogenic Archaea. Shifts in microbial community abundance (DAPI counts) and composition (Illumina-MiSeq and FISH analyses) were observed.
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Effects of Increasing Concentrations of Enrofloxacin on Co-Digestion of Pig Manure and Corn Straw. SUSTAINABILITY 2022. [DOI: 10.3390/su14105894] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Enrofloxacin (ENR) is one of the most commonly used antibiotics in pig farms. In this study, using fresh pig manure and corn straw powder as substrates, the effects of different concentrations of ENR (2.5, 10, and 20 mg/L) on anaerobic digestion in completely mixed anaerobic reactors were investigated. A relatively low concentration of ENR (2.5 mg/L) increased methane production by 47.58% compared with the control group. Among the volatile fatty acids (VFAs) in the reactors, the propionic acid content was the lowest, and the concentrations of acetic acid kinase and coenzyme F420 were highest in the first seven days during peak gas production. However, methane production in the reactors with 10 mg/L and 20 mg/L ENR decreased by 8.59% and 20.25%, respectively. Furthermore, the accelerated hydrolysis of extracellular polymeric substances causes a significant accumulation of VFA levels. The microbial community in anaerobic reactors was analyzed by 16S rRNA sequencing. Proteiniphilum was the dominant bacterial genus. In addition, ENR at 2.5 mg/L effectively increased the abundance and diversity of anaerobic microorganisms, whereas a high concentration of ENR (10 and 20 mg/L) significantly decreased these parameters. This study demonstrated that different concentrations of ENR had significantly different effects on anaerobic digestion.
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Mushtaq M, Zeeshan M, Nawaz I, Hassan M. Effect of low levels of oxytetracycline on anaerobic digestion of cattle manure. BIORESOURCE TECHNOLOGY 2022; 349:126894. [PMID: 35217155 DOI: 10.1016/j.biortech.2022.126894] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2021] [Revised: 02/16/2022] [Accepted: 02/18/2022] [Indexed: 06/14/2023]
Abstract
Cattle manure is rich in organic matter and nutrients, but it may also contain harmful substances such as residual antibiotics and other toxic compounds. Oxytetracycline (OTC) is a widely used veterinary antibiotic and its presence in manure can affect the subsequent anaerobic digestion process. This study evaluated the effect of OTC concentrations viz. 0.12, 0.3, 0.6, 1.2, 3, 6 and 12 mg L-1 on batch mesophilic anaerobic digestion of cattle manure. The results showed that cumulative biogas yield decreased by 25, 29 and 55% at 3, 6 and 12 mg OTC L-1 in contrast to control. Volatile solids removal was 39% for control which reduced to 13% in 12 mg L-1 OTC spiked reactor. Effect on stability parameters was significant at OTC concentrations from 1.2 to 12 mg L-1. Two different kinetic models were used for biogas data validation and the modified Gompertz model best fitted to the experimental data.
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Affiliation(s)
- Moniba Mushtaq
- Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan
| | - Muhammad Zeeshan
- Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan
| | - Ismat Nawaz
- Department of Biosciences, COMSATS University Islamabad, Islamabad Campus, Islamabad, Pakistan
| | - Muneeb Hassan
- Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan
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Metagenomic Analysis of the Long-Term Synergistic Effects of Antibiotics on the Anaerobic Digestion of Cattle Manure. ENERGIES 2022. [DOI: 10.3390/en15051920] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The conversion of cattle manure into biogas in anaerobic digestion (AD) processes has been gaining attention in recent years. However, antibiotic consumption continues to increase worldwide, which is why antimicrobial concentrations can be expected to rise in cattle manure and in digestate. This study examined the long-term synergistic effects of antimicrobials on the anaerobic digestion of cattle manure. The prevalence of antibiotic resistance genes (ARGs) and changes in microbial biodiversity under exposure to the tested drugs was investigated using a metagenomic approach. Methane production was analyzed in lab-scale anaerobic bioreactors. Bacteroidetes, Firmicutes, and Actinobacteria were the most abundant bacteria in the samples. The domain Archaea was represented mainly by methanogenic genera Methanothrix and Methanosarcina and the order Methanomassiliicoccales. Exposure to antibiotics inhibited the growth and development of methanogenic microorganisms in the substrate. Antibiotics also influenced the abundance and prevalence of ARGs in samples. Seventeen types of ARGs were identified and classified. Genes encoding resistance to tetracyclines, macrolide–lincosamide–streptogramin antibiotics, and aminoglycosides, as well as multi-drug resistance genes, were most abundant. Antibiotics affected homoacetogenic bacteria and methanogens, and decreased the production of CH4. However, the antibiotic-induced decrease in CH4 production was minimized in the presence of highly drug-resistant microorganisms in AD bioreactors.
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