Influence of lincomycin on anaerobic digestion: Sludge type, biogas generation, methanogenic pathway and resistance mechanism

Mechanism of biochar in alleviating the inhibition of anaerobic digestion under ciprofloxacin press

The antibiotic ciprofloxacin (CIP), detected in various aqueous environments, has broad-spectrum antimicrobial properties that can severely affect methanogenic performance in anaerobic systems. In this study, a novel strategy to alleviate the inhibition of AD performance under CIP press with the direct addition of biochar (BC) prepared from corn stover was proposed and the corresponding alleviation mechanism was investigated. When the dosage of BC was 5 and 20 g/L, the cumulative methane production in AD could reach 317.9 and 303.0 mL/g COD, and the CIP degradation efficiencies reached 94.1 % and 96.6 %, significantly higher than those of 123.0 mL/g COD and 81.2 % in the Control system. BC avoided excessive reactive oxygen species in anaerobic systems and induced severe oxidative stress response, while protecting the cell membrane and cell wall of microorganisms. Microorganisms could consume and utilize more organic extracellular polymeric substances for their growth and metabolism. When BC was involved in AD, fewer toxic intermediates were generated during CIP biodegradation, reducing acute and chronic toxicity in anaerobic systems. Microbial diversity suggested that BC could enrich functional microorganisms involved in direct interspecies electron transfer like Methanosaeta, norank_f_Bacteroidetes_vadinHA17, JGI-0000079-D21 and Syntrophomonas, thus facilitating the methanogenic process and CIP degradation. Genetic analyses showed that BC could effectively upregulate functional genes related to the conversion of butyrate-to-acetate and acetyl-to-methane under CIP stress, while functional gene abundance associated with CIP degradation enhanced partially, about encoding translocases, oxidoreductases, lyases, and ligases. Therefore, BC can be added to AD under CIP press to address its inhibited methanogenic performance.

The contribution of swine wastewater on environmental pathogens and antibiotic resistance genes: Antibiotic residues and beyond

Swine wastewater application can introduce antibiotics, antibiotic resistance genes (ARGs) into environments. Herein, the full-scale transmission of antibiotics, ARGs and their potential carriers from an intensive swine feedlot to its surroundings were explored. Results showed that lincomycin and doxycycline hydrochloride were dominant antibiotics in this ecosystem. Lincomycin concentration were strongly associated with soil bacterial communities. According to the risk quotient (RQ), lincomycin was identified as posing higher ecological risk in aquatic environments. ARGs and mobile genetic elements (MGEs) abundance in wastewater were reduced after anaerobic treatment. Notably, ARGs composition of environmental samples were clustered into two groups based on if they were directly affected by the wastewater. However, there were no remarkable difference of ARGs abundance among environmental samples. The total abundance of ARGs was positively related to that of MGEs. Pathogens Escherichia coli and Enterococcus revealed strong connection with qnrS, tet and sul. Overall, this study highlights the importance of responsible antibiotics use in livestock production and appropriate treatment technology before agricultural application and discharge.

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.

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.

Underlying the inhibition mechanisms of sulfate and lincomycin on long-term anaerobic digestion: Microbial response and antibiotic resistance genes distribution

This study evaluated the resilience of a long-term anaerobic treatment system exposed to sulfate, lincomycin (LCM) and their combined stress. LCM was found to impede anaerobic propionate degradation, while sulfate for restraining methanogenic acetate utilization. The combined stress, with influent LCM of 200 mg/L and sulfate of 1404 mg/L, revealed severer inhibition on anaerobic digestion than individual inhibition, leading to 73.9 % and 38.5 % decrease in methane production and sulfate removal, respectively. Suppression on propionate-oxidizing bacteria like unclassified_f__Anaerolineae and unclassified_f__Syntrophaceae further demonstrated LCM's inhibitory effect on propionate degradation. Besides, the down-regulation of genes encoding dissimilatory sulfate reduction enzymes caused by LCM triggered great inhibition on sulfate reduction. A notable increase in ARGs was detected under sulfate-stressed condition, owing to its obvious enrichment of tetracycline-resistant genes. Genera including unclassified_f__Syntrophaceae, unclassified_f__Geobacteraceae and unclassified_f__Anaerolineaceae were identified as dominant host of ARGs and enriched by sulfate addition. Overall, these results could provide the theoretical basis for further enhancement on anaerobic digestion of pharmaceutical wastewater containing sulfate and lincomycin.

Assessing the fate of antibiotics and agrochemicals during anaerobic digestion of animal manure

Antibiotics and pesticides are used extensively by the livestock industry. Agricultural chemicals can pose potential human and environmental health risks due to their toxicity and through their contributions to antimicrobial resistance, and strategies to reduce their emission into the environment are urgently needed. Anaerobic digestion (AD) is a sustainable technology for manure management that produces biogas while also providing an opportunity to degrade agricultural chemicals that are present in manure. While the effects of selected chemicals on biogas production have been investigated previously, little is known about chemical transformations during AD. Using lab-scale AD batch reactors containing dairy manure, degradation kinetics and transformation products (TPs) were investigated for twenty compounds that are likely to be present in manure management systems and that we hypothesized would transform during AD. Digestate samples were extracted using a modified quick, easy, cheap, effective, rugged, and safe (QuEChERS) method and analyzed using liquid chromatography - high-resolution mass spectrometry. Eleven of the tested chemicals degraded, leading to the formation of 47 TPs. Three compounds degraded abiotically only, two degraded biotically only, and six degraded both abiotically and biotically. These results suggest that in addition to renewable energy generation, AD contributes to the degradation of chemical contaminants present in agricultural waste streams. However, the potential toxic effects of TPs require further investigation.

A critical review on retaining antibiotics in liquid digestate: Potential risk and removal technologies

Substantial levels of antibiotics remain in liquid digestate, posing a significant threat to human safety and the environment. A comprehensive assessment of residual antibiotics in liquid digestate and related removal technologies is required. To this end, this review first evaluates the potential risks of the residual antibiotics in liquid digestate by describing various anaerobic digestion processes and their half-lives in the environment. Next, emerging technologies for removing antibiotics in liquid digestate are summarized and discussed, including membrane separation, adsorption, and advanced oxidation processes. Finally, this study comprehensively and critically discusses these emerging technologies' prospects and challenges, including techno-economic feasibility and environmental impacts.

Effect of anthraquinone-2,6-disulfonate (AQDS) on anaerobic digestion under ammonia stress: Triggering mediated interspecies electron transfer (MIET)

The underlying mechanisms by which humic-like substrates affect anaerobic digestion under ammonia stress are insufficiently understood so far. In this study, anthraquinone-2,6-disulfonate (AQDS), a representative analogue of humic acid, was adopted at a 100 μM concentration as the exogenous additive during anaerobic digestion process along with 5.0 g NH₄⁺-N/L stress. The results showed that AQDS could improve the cumulative CH₄ production and the maximum CH₄ production rate by 7.3 and 10.8%, respectively, and shorten the methanogenic lag phase by 13.8%. Acetate-related production and methanation were both facilitated, during which the biological rather than the chemical mechanism played a crucial role. The microbial diversity distribution revealed that electroactive Anaerolinea and Methanosaeta were significantly enriched in response to AQDS amendment. Herein, AQDS was presumed to serve as an electron shuttle to trigger a mediated interspecies electron transfer (MIET) network among electroactive consortia, thus accelerating acetate methanation and ameliorating methanogenesis under ammonia stress.

Biochar assisted cellulose anaerobic digestion under the inhibition of dodecyl dimethyl benzyl ammonium chloride: Dose-response kinetic assays, performance variation, potential promotion mechanisms

This study evaluated the inhibitory effect and mitigation strategy of dodecyl dimethyl benzyl ammonium chloride (DDBAC) suppression on anaerobic digestion. With the 12 h-suppression, only 16.64% of anaerobes were alive, and acetotrophic methanogens were significantly inhibited. As for batch test, DDBAC suppression significantly prolonged the start-up of systems and decreased the biogas production. In cellulose semi-continuous digestion process, the DDBAC suppression induced volatile fatty acids accumulation and pH decrease. However, the biochar amended reactor effectively mitigated the DDBAC suppression and achieved 370.5 mL/d·g-chemical-oxygen-demand biogas production. Moreover, 17.8% more protein in extracellular polymeric substances was secreted as the bio-barrier to defense the DDBAC suppression. Furthermore, microbial analysis showed that biochar addition selectively enriched directed interspecies electron transfer (DIET) participant bacteria (Anaerolineaceae and Syntrophomonas) and methanogens (Methanosaeta and Methanobacterium). Meanwhile, the potential metabolic pathway analysis showed that the abundance of amino acids and energy metabolism were increased 28% and 8%, respectively. The abundance of encoding enzyme related to hydrogenotrophic and acetotrophic methanogenesis enriched 1.88 times and 1.48 times, respectively. These results showed the performance and mechanisms involved in DIET establishment with ethanol stimulation biochar addition.

Influence of sulfamethoxazole on anaerobic digestion: Methanogenesis, degradation mechanism and toxicity evolution

Sulfamethoxazole (SMX), one of the most widely used sulfonamides antibiotics, is frequently detected in the livestock wastewater. Currently, the focus needs to shift from performance effects to understanding of mechanisms and intermediate toxicity analysis. Our study found that SMX (0.5, 1, and 2 mg/L) stimulated methane production by promoting the process of acetogenesis and homo-acetogenesis. Since 1 mg/L SMX could inhibit the transformation of butyric acid, thus, the stimulation of methane was weak under this condition. Under anaerobic conditions, acetate kinase (AK) and cytochrome P450 enzymes (CYP450) continued to participate in SMX degradation. The increase in SMX concentration affected the release of metabolic enzymes, causing changes in SMX degradation pathways. Based on the main biotransformation products, five biotransformation pathways were proposed, the major transformation reactions including hydroxylation, hydrogenation, acetylation, deamination, oxidation, the elimination of oxygen atoms on sulfonyl, isoxazole ring and NS bond cleavage. Toxicity prediction analysis showed that the toxicities of most SMX transformation products were lower than that of SMX.

Analysis of organic matter conversion behavior and kinetics during thermal hydrolysis of sludge and its anaerobic digestion performance

In thermal hydrolysis (TH) of waste activated sludge (WAS), the material transformation of a specific temperature heating for a set duration is generally examined. However, this study looked at the material changes of TH as the temperature rose (90-210 °C) and the kinetic derivation of soluble chemical oxygen demand (SCOD), protein, and carbohydrate using the Coats-Redfern model. It was found that the proportion of soluble protein and soluble carbohydrate in the organic components and their contents reached the maximum (17.39 and 8.10 g L^-1 respectively) at 180 °C. Differently, volatile fatty acid (VFA), amino acids, and ammonia nitrogen increased with the TH temperature and reached a maximum at 210 °C. The fitting equations of non-isothermal dynamics at the medium- and low-temperature stages (90-180 °C) at n = 1, 0.5, and 2 were studied. When n = 1, the activation energies of COD, protein, and carbohydrate were 33.32, 23.34, and 36.15 kJ mol^-1, respectively. And the kinetic analysis results were in good agreement with the experimental results (the maximum rate of increase in protein and carbohydrate was at 135-150 °C and 150-180 °C, respectively). Moreover, the pattern of anaerobic digestion performance of WAS was comparable to the trend of protein and carbohydrate in TH, the highest cumulative methane production was 159.68 mL·g^-1VS for the TH sludge at 180 °C. This study provided a theoretical foundation for the use of thermal hydrolysis in engineering.

Multi-faceted influences of biochar addition on swine manure digestion under tetracycline antibiotic pressure

This study explored the influence of biochar (BC) on anaerobic digestion (AD) of swine manure under various tetracycline (TC) pressures. It was found that both low (0.5 mg/L) and high (50 mg/L) TC pressures inhibited AD performance, while BC mitigated it in multi-facets. Under high TC pressure, BC accelerated syntrophic methanogenesis by boosting direct interspecies electron transfer pathway. The TC removal efficiencies were enhanced by 24.3-158.2% with BC assistance, which was attributed to the enhanced biological degradation rather than BC's physiochemical adsorption. Moreover, BC possibly acted as a protective role to alleviate intensive extracellular polymeric substances secretion under TC pressures. Integrated microbial community, metabolic function predicting, and antibiotic resistance genes (ARG) analysis revealed that BC addition not only enriched Anaerolineceae, which likely responsible for the 24.2-41.9% higher level expression of organics metabolic pathways and xenobiotics biodegradation, but also reduced ARG abundance by controlling the potential ARG host (Firmicutes) proliferation.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.