Marine biodegradation of plastic films by Alcanivorax under various ambient temperatures: Bacterial enrichment, morphology alteration, and release of degradation products.

The global ocean has been receiving massive amounts of plastic wastes. Marine biodegradation, influenced by global climate, naturally breaks down these wastes. In this study, we systematically compared the biodegradation performance of petroleum- and bio-based plastic films, i.e., low-density polyethylene (LDPE), polylactic acid (PLA), and polyhydroxyalkanoates (PHAs) under three ambient temperatures (4, 15, and 22 °C). We deployed the our previously isolated cold-tolerant plastic-degrading Alcanivorax to simulate the accelerated marine biodegradation process and evaluated the alteration of bacterial growth, plastic films, and released degradation products. Notably, we found that marine biodegradation of PHA films enriched more bacterial amounts, induced more conspicuous morphological damage, and released more microplastics (MPs) and dissolved organic carbon (DOC) under all temperatures compared to LDPE and PLA. Particularly, MPs were released from film edges and cracks with a mean size of 2.8 µm under all temperatures. In addition, the degradation products released by biodegradation of PHA under 22 °C induced the highest acute toxicity to Vibrio fischeri. Our results highlighted that: (1) marine biodegradation of plastics would release millions of MPs per cm2 exposed surface area even in cold environments within 60 days; (2) different marine biodegradation scenarios of these plastics may raise disparate impacts and mitigation-related studies.

Analysis of death cases in Shenyang City, China, for immunization adverse event surveillance, 2009-2021.

Through the Chinese National Immunization Adverse Event Surveillance System (CNAEFIS), we collected reports of Adverse Event Following Immunization (AEFI) deaths in Shenyang from 2009 to 2021 with the aim of analyzing AEFI-related deaths and assessing the safety of vaccination. From 2009 to 2021, a total of 12 AEFI-related deaths were reported in Shenyang City, and autopsies were performed in 6 deaths. According to the assessment of the Expert Committee on Investigation and Diagnosis of AEFI 3 (25.0%) deaths were classified as severe vaccine reactions, 9 (75.0%) deaths were classified as coincidental events, and there were no immunization errors or psychological reactions. The overall estimated AEFI-related mortality rate was 0.12 per 100,000 vaccination doses. Spearman's rank correlation analysis showed no correlation between AEFI, severe vaccine reactions, and suspected vaccination-related deaths. Coincidental events are the most common type of death following vaccination, meaning that the risk of death following immunization is low, and ongoing AEFI surveillance and scientific causality assessment are essential to ensure the vaccine confidence. Detailed pre-vaccination health status questioning is also key to avoiding and reducing adverse events.

Micropollutants but high risks: Human multiple stressors increase risks of freshwater ecosystems at the megacity-scale.

Micropollutants in water environments have attracted widespread attention, but how human and natural stressors influence the risks of micropollutants has not been comprehensively revealed. A megacity-scale study of the ecological risks of micropollutants in the surface water of Beijing, China is presented to illustrate the magnitudes of the influences of multiple anthropogenic and natural stressors. A total of 133 micropollutants representing typical land use patterns in Beijing, were quantified with the mean concentration range of ND (not detected) to 272 ng·L-1. The micropollutant concentrations in the south were obviously higher than those detected in the northern areas, and neonicotinoid pesticides showed the highest mean concentration of 311 ng·L-1. The chronic and acute risks of micropollutants to algae, invertebrates, and fishes were determined, and herbicides, organophosphorus esters, and insecticides account for the primary risks to algae, invertebrates, and fishes, respectively. The cropland and impervious cover cause the differences in the pollution and risks of micropollutants. The land use in riparian zones greater than 2 km shows a great influence on the chronic chemical risks (CCRs) for the three groups of species, indicating that too local scale does not explain the local pollution status. Climate conditions and human land use are important drivers explaining the CCRs to which various trophic levels of species are exposed. Results demonstrate that multiple categories of micropollutants pose adverse risks to freshwater in the megacity of Beijing, while climate conditions, pollution discharge, and human land use induce the chemical risk of micropollutants to aquatic organisms, and the land use in different riparian zones show different effects on the risks.

Comparison of O3, UV/O3, and UV/O3/PS processes for marine oily wastewater treatment: Degradation performance, toxicity evaluation, and flocs analysis.

Efficient on-site treatment technology is crucial for mitigating marine oily wastewater pollution. This work investigates the ozone (O3), ultraviolet (UV)/O3, UV/O3/persulfate (PS) processes for the treatment of marine oily wastewater, including degradation performance, acute toxicity evaluation, and oil flocs analysis in a benchtop circulating flow photoozonation reactor. Degradation performances have been studied by measuring the degradation rate of total oil concentrations, specific oil components (n-alkanes and polycyclic aromatic hydrocarbons (PAHs)), and total organic carbon (TOC). The results show that UV/O3/PS could significantly enhance the removal efficiency than the other two processes, with above 90% of removal efficiency in 30 min. Acute toxicity analysis further shows that the wastewater quality is significantly improved by four-fold of the EC50 of Vibrio fischeri, and the mortality of Artemia franciscana decreases from 100% to 0% after 48 h exposure. Further, the morphology and functional groups of flocs have been further characterized, showing that the floating flocs could be further degraded especially in UV/O3/PS process. Our study further raised discussions regarding the future on-site application of O3-based systems, based on the results generated from the treatment efficiency, toxicity, and flocs characterization. The regulation of the oxidation strength and optimization of the reaction systems could be a practical strategy for on-site marine oily wastewater treatment.

Responses of Alcanivorax species to marine alkanes and polyhydroxybutyrate plastic pollution: Importance of the ocean hydrocarbon cycles.

Understanding microbial responses to hydrocarbon and plastic pollution are crucial for limiting the detrimental impacts of environmental contaminants on marine ecosystems. Herein, we reported a new Alcanivorax species isolated from the North Atlantic Ocean capable of degrading alkanes and polyhydroxybutyrate (PHB) plastic (one of the emerging bioplastics that may capture the future plastic market). The whole-genome sequencing showed that the species harbors three types of alkane 1-monooxygenases (AlkB) and one PHB depolymerase (PhaZ) to initiate the degradation of alkanes and plastics. Growth profiling demonstrated that n-pentadecane (C15, the main alkane in the marine environment due to cyanobacterial production other than oil spills) and PHB could serve as preferential carbon sources. However, the cell membrane composition, PhaZ activity, and expression of three alkB genes were utterly different when grown on C15 and PHB. Further, Alcanivorax was a well-recognized alkane-degrader that participated in the ocean hydrocarbon cycles linking with hydrocarbon production and removal. Our discovery supported that the existing biogeochemical processes may add to the marine ecosystem's resilience to the impacts of plastics.

Development of advanced oil/water separation technologies to enhance the effectiveness of mechanical oil recovery operations at sea: Potential and challenges.

Mechanical oil recovery (i.e., booming and skimming) is the most common tool for oil spill response. The recovered fluid generated from skimming processes may contain a considerable proportion of water (10 % ~ 70 %). As a result of regulatory prohibition on the discharge of contaminated waters at sea, vessels and/or storage barges must make frequent trips to shore for oil-water waste disposal. This practice can be time- consuming thus reduces the overall efficiency and capacity of oil recovery. One potential solution is on-site oil-water separation and disposal of water fraction at sea. However, currently available decanting processes may have limited oil/water separation capabilities, especially in the presence of oil-water emulsion, which is inevitable in mechanical oil recovery. The decanted water may not meet the discharge standards and cause severe ecotoxicological impacts. This paper therefore comprehensively reviews the principles and progress in oil/water separation, demulsification, and on-site treatment technologies, investigates their applicability on decanting at sea, and discusses the ecotoxicity of decanted water in the marine environment. The outputs provide the fundamental and practical knowledge on decanting and help enhance response effectiveness and consequently reducing the environmental impacts of oil spills.

Metagenomic and Metatranscriptomic Responses of Chemical Dispersant Application during a Marine Dilbit Spill.

The global increase in marine transportation of dilbit (diluted bitumen) can increase the risk of spills, and the application of chemical dispersants remains a common response practice in spill events. To reliably evaluate dispersant effects on dilbit biodegradation over time, we set large-scale (1,500 mL) microcosms without nutrient addition using a low dilbit concentration (30 ppm). Shotgun metagenomics and metatranscriptomics were deployed to investigate microbial community responses to naturally and chemically dispersed dilbit. We found that the large-scale microcosms could produce more reproducible community trajectories than small-scale (250 mL) ones based on the 16S rRNA gene amplicon sequencing. In the early-stage large-scale microcosms, multiple genera were involved in the biodegradation of dilbit, while dispersant addition enriched primarily Alteromonas and competed for the utilization of dilbit, causing depressed degradation of aromatics. The metatranscriptomic-based metagenome-assembled genomes (MAG) further elucidated early-stage microbial antioxidation mechanism, which showed that dispersant addition triggered the increased expression of the antioxidation process genes of Alteromonas species. Differently, in the late stage, the microbial communities showed high diversity and richness and similar compositions and metabolic functions regardless of dispersant addition, indicating that the biotransformation of remaining compounds can occur within the post-oil communities. These findings can guide future microcosm studies and the application of chemical dispersants for responding to a marine dilbit spill. IMPORTANCE In this study, we employed microcosms to study the effects of marine dilbit spill and dispersant application on microbial community dynamics over time. We evaluated the impacts of microcosm scale and found that increasing the scale is beneficial for reducing community stochasticity, especially in the late stage of biodegradation. We observed that dispersant application suppressed aromatics biodegradation in the early stage (6 days), whereas exerting insignificant effects in the late stage (50 days), from both substance removal and metagenomic/metatranscriptomic perspectives. We further found that Alteromonas species are vital for the early-stage chemically dispersed oil biodegradation and clarified their degradation and antioxidation mechanisms. These findings help us to better understand microcosm studies and microbial roles for biodegrading dilbit and chemically dispersed dilbit and suggest that dispersant evaluation in large-scale systems and even through field trails would be more realistic after marine oil spill response.

Advanced oxidation processes in microreactors for water and wastewater treatment: Development, challenges, and opportunities.

The miniaturization of reaction processes by microreactors offers many significant advantages over the use of larger, conventional reactors. Microreactors' interior structures exhibit comparatively higher surface area-to-volume ratios, which reduce reactant diffusion distances, enable faster and more efficient heat and mass transfer, and better control over process conditions. These advantages can be exploited to significantly enhance the performance of advanced oxidation processes (AOPs) commonly used for the removal of water pollutants. This comprehensive review of the rapidly emerging area of environmental microfluidics describes recent advances in the development and application of microreactors to AOPs for water and wastewater treatment. Consideration is given to the hydrodynamic properties, construction materials, fabrication techniques, designs, process features, and upscaling of microreactors used for AOPs. The use of microreactors for various AOP types, including photocatalytic, electrochemical, Fenton, ozonation, and plasma-phase processes, showcases how microfluidic technology enhances mass transfer, improves treatment efficiency, and decreases the consumption of energy and chemicals. Despite significant advancements of microreactor technology, organic pollutant degradation mechanisms that operate during microscale AOPs remain poorly understood. Moreover, limited throughput capacity of microreactor systems significantly restrains their industrial-scale applicability. Since large microreactor-inspired AOP systems are needed to meet the high-throughput requirements of the water treatment sector, scale-up strategies and recommendations are suggested as priority research opportunities. While microstructured reactor technology remains in an early stage of development, this work offers valuable insight for future research and development of AOPs in microreactors for environmental purposes.

Machine learning-aided causal inference for unraveling chemical dispersant and salinity effects on crude oil biodegradation.

Chemical dispersants have been widely applied to tackle oil spills, but their effects on oil biodegradation in global aquatic systems with different salinities are not well understood. Here, both experiments and advanced machine learning-aided causal inference analysis were applied to evaluate related processes. A halotolerant oil-degrading and biosurfactant-producing species was selected and characterized within the salinity of 0-70 g/L NaCl. Notably, dispersant addition can relieve the biodegradation barriers caused by high salinities. To navigate the causal relationships behind the experimental data, a structural causal model to quantitatively estimate the strength of causal links among salinity, dispersant addition, cell abundance, biosurfactant productivity and oil biodegradation was built. The estimated causal effects were integrated into a weighted directed acyclic graph, which showed that overall positive effects of dispersant addition on oil biodegradation was mainly through the enrichment of cell abundance. These findings can benefit decision-making prior dispersant application under different saline environments.

Microplastic-oil-dispersant agglomerates in the marine environment: Formation mechanism and impact on oil dispersion.

Microplastics (MPs) can interact with spilled oil to form MP-oil-dispersant agglomerates (MODAs) in oceans. This study investigated the MODA formation mechanism and its impact on oil dispersion during marine oil spill responses. Two types of agglomerates, MODA-1 (MP-in-oil) and MODA-2 (MP-oil droplet-embedded), were identified. The 12 µm-MPs only formed MODA-1, while 45 µm-MPs and 125 µm-MPs formed MODA-1 and MODA-2 due to the surface free energy minimization principle. Impacts of MODA on oil dispersion under different mixing energy levels and seawater salinities were explored. We found that MODA reduced oil dispersion effectiveness under different mixing energy levels. Among three MP sizes, 12 µm-MPs caused the greatest reduction in dispersion effectiveness due to the formation of MODA-1. Pristine 12 µm-MPs reduced dispersion effectiveness by 21.95% under 5.62 × 10-1 W/kg, while pristine 45 µm-MPs and pristine 125 µm-MPs decreased it by 5.85% and 1.83%, respectively. In addition, MODA formed by pristine MPs has a larger impact on oil dispersion effectiveness than that of aged MPs under different salinities. Under 20psu, pristine 12 µm-MPs reduced dispersion effectiveness by 33.68%, while aged 12 µm-MPs decreased it by 24.61%. This study is the first report on the MODA formation mechanism, which is essential for exploring MODA transport and toxicity through marine trophic levels.

Access-dispersion-recovery strategy for enhanced mitigation of heavy crude oil pollution using magnetic nanoparticles decorated bacteria.

Heavy crude oil (HCO) pollution has gained global attention, but traditional bioremediating practices demonstrate limited effectiveness. This study developed magnetic nanoparticles decorated bacteria (MNPB) using an oil-degrading and biosurfactant-producing Rhodococcus erythropolis species and identified a novel access-dispersion-recovery strategy for enhanced HCO pollution mitigation. The strategy entails (1) magnetic navigation of the MNPB towards HCO layer, (2) enhanced oil dispersion and formation of suspended oil-bacteria aggregates, and (3) magnetic recovery of these aggregates. The UV-spectrophotometer analysis showed that this strategy can enable up to 62% removal of HCO. The GC-MS analysis demonstrated that the MNPB enhanced the degradation of low-molecular-weight aromatics comparing with the pure bacteria, and the recovery process further removed oil-bacteria aggregates and entrained high-molecular-weight aromatics. The feasibility of using MNPB to mitigate HCO pollution could shed light on the emerging bioremediation applications.

Interactions between microplastics and oil dispersion in the marine environment.

Microplastics (MPs) and spilled oil are both major concerns in the marine environment. In this study, we investigated if and how MPs would interact with crude oil and potentially reduce the effectiveness of oil dispersants applied during oil spill response operations. With the addition of dispersant, MPs and oil (covered by dispersants through their hydrophobic tails) formed MPs-oil-dispersant agglomerates that were found to exist from the surface layer to the bottom of the seawater column. Their resurfacing and sinking led to a decrease in oil dispersion effectiveness. Effects of MP concentration, MP aging, and dispersant-to-oil volumetric ratios (DORs) on oil dispersion were examined. We found that the dispersion effectiveness of light oil and heavy oil decreased 38.26 % and 38.25 %, respectively, with an increased MP concentration. The dispersion effectiveness of light oil and heavy oil was 82.86 ± 10.87 % and 40.39 ± 4.96 % with pristine MPs and increased up to 109.75 ± 0.71 % and 58.30 ± 0.00 % when using MPs aged for 56 days. MPs reduced oil dispersion effectiveness under different DORs. The findings of this first report to understand the interactions among MPs, oil and dispersants have provided fundamental insights that may influence future decision making on the selection and use of oil spill response strategies.

Level, Source, and Spatial Distribution of Potentially Toxic Elements in Agricultural Soil of Typical Mining Areas in Xiangjiang River Basin, Hunan Province.

The concentrations, chemical availability, distribution, and sources of potentially toxic elements (PTEs) in the soil of Xiangjiang Basin in Hunan Province, China were investigated at 85 sites. The highest mean concentrations of Cd, Cu, Zn, As, and Pb were observed in Hengyang, whereas those for Mn, Co, and Hg were observed in Changde. The pollution index values followed the order: Cd > Hg > Cu > Zn > As > Pb; the mean geo-accumulation index values were in the order: Cd > Hg > Pb > Cu > Zn > As > Co > Mn. Cd was associated with moderate contaminated level, Hg and Pb were associated with moderate contaminated to uncontaminated level, and Cu, Zn, As, Co, and Mn were associated with uncontaminated level of pollution. Furthermore, 64.5% of Cd was water-soluble and exhibited exchangeable fractions; its chemical availability posed a risk to the ecosystem. Spatial analysis, principal component analysis, and a positive matrix factorization model were used to assess the PTE sources. Four principal components contributed to 88.8% of the 8 PTEs concentrations. Mining, smelting, industrial, and agricultural activities, alongside sewage irrigation, the use of agrochemicals, and vehicular emissions are the possible anthropogenic sources that pollute agricultural products and threaten human health in the Xiangjiang Basin.

Microbial eco-physiological strategies for salinity-mediated crude oil biodegradation.

Salinity variability strongly affects the behaviors of oil degrading bacteria for spilled oil biodegradation in the marine environment. However, limited studies explored the strategies of microbes on salinity-mediated crude oil biodegradation. In this study, a halotolerant bio-emulsifier producer, Exiguobacterium sp. N41P, was examined as a model strain for Alaska North Slope (ANS) crude oil (0.5%, v/v) biodegradation. Results indicated that Exiguobacterium sp. N41P could tolerant a wide range of salinity (0-120 g/L NaCl) and achieve the highest degradation efficiency under the salinity of 15 g/L NaCl due to the highest biofilm formation ability. Moreover, increased salinity induced decreased cell surface hydrophobicity and a migration of microbial growth from oil phase to aqueous phase, leading to limited bio-emulsifier productivity and depressed degradation of insoluble long-chain n-alkanes while enhancing the degradation of relative soluble naphthalene. Research findings illustrated the microbial eco-physiological mechanism for spilled oil biodegradation under diverse salinities and advanced the understanding of sophisticated marine crude oil biodegradation process.

Digital PCR as an Emerging Tool for Monitoring of Microbial Biodegradation.

Biodegradation of contaminants is extremely complicated due to unpredictable microbial behaviors. Monitoring of microbial biodegradation drives us to determine (1) the amounts of specific degrading microbes, (2) the abundance, and (3) expression level of relevant functional genes. To this endeavor, the cultivation independent polymerase chain reaction (PCR)-based monitoring technique develops from endpoint PCR, real-time quantitative PCR, and then into novel digital PCR. In this review, we introduce these three categories of PCR techniques and summarize the timely applications of digital PCR and its superiorities than qPCR for biodegradation monitoring. Digital PCR technique, emerging as the most accurately absolute quantification method, can serve as the most promising and robust tool for monitoring of microbial biodegradation.

Landscape Pattern Changes in the Xingkai Lake Area, Northeast China.

Understanding landscape change is important for ecologically sustainable development. In this paper, we assessed the spatiotemporal variations of landscape pattern in the Xingkai Lake area using remote sensing data from 1982, 1995, 2000, 2005, 2010, and 2015. Landscape patterns of marshlands, paddy fields, dry farmlands, and their combinations were analyzed at class and landscape levels. We examined the stability of landscape types through principal component analysis based on class level indices for landscape types. The results indicated that marshland areas decreased significantly by 33.87% but paddy fields increased by 1.84 times from 1982 to 2015. The largest conversion of dry farmlands to paddy fields was 90.88 km2 during the period 2010-2015. In contrast, the largest conversion of paddy fields to dry farmlands was 86.03 km2 during the period 2000-2005. The difference in relative change revealed that dry farmlands had experienced a greater relative change than paddy fields since 2000. The interspersion and juxtaposition index decreased, while the number of patches grew. This showed that landscape fragmentation was increasing and the landscape pattern was becoming dispersed. Marshlands were more stable than paddy fields and dry farmlands across all time periods, except for the year 2005.

Preparation, adsorptive properties and chemical regeneration studies of high-porous activated carbon derived from Platanus orientalis leaves for Cr(VI) removal.

Activated carbon (AC) was prepared from Platanus orientalis leaves by H3PO4 activation using a microwave heating method and characterized by SEM (scanning electron microscopy), Brunauer-Emmett-Teller (BET) surface area analysis and FTIR (Fourier transform infrared spectroscopy) techniques. AC exhibited a surface area of 1089.67 m2/g and a relatively high pore volume of 1.468 cm3/g. Utilization of AC for the removal of Cr(VI) from aqueous solution was researched. The adsorption efficiency was highly pH dependent and adsorption capacity of AC for Cr(VI) could reach up to 135.24 mg/g. Adsorption equilibrium could be quickly reached within 2 h. A kinetic study indicated that the adsorption of Cr(VI) conformed to the pseudo-second-order model (R2 > 0.99). An intraparticle diffusion model was applied to describe the adsorption kinetics, and the results showed that there are other factors that affect the rate. Chemical regeneration for AC saturated with Cr(VI) was performed and HNO3 displayed the best regeneration performance among the four chemical regeneration agents (HNO3, H2SO4, NaOH, NaCl). The regeneration performance increased at first and then decreased with the rise of HNO3 concentration, and regeneration reaction could reach equilibrium within 4 h in the first cycle. The FTIR spectra revealed that HNO3 successfully introduced N-H bonds onto the AC surface in the regeneration process.

Bi2Zr2O7 nanoparticles synthesized by soft-templated sol-gel methods for visible-light-driven catalytic degradation of tetracycline.

Tetracycline (TC), an antibiotic, is persistent in nature and frequently detected in water and sediments. Visible-light-driven photocatalyst for TC degradation is a promising environmental-friendly technology. Bi2Zr2O7, an effective photocatalyst, but no studies on its photodegradation of TC could be found in literature. In this study, Bi2Zr2O7 was synthesized by three soft templated sol-gel methods. Three synthesized Bi2Zr2O7 catalysts have different structures, morphologies and band gaps. The Bi2Zr2O7 nanoparticles synthesized with citric acid as the template (BZO-3) had a larger specific surface area (30.7 m2/g) and a narrower band gap (2.39 eV), and exhibited a better performance for TC degradation under visible light with an efficiency of up to 81.3%. They also exhibited good stability and reusability in recycled experiments. A reaction mechanism of TC degradation by these photocatalyst was proposed. The enhanced photocatalytic performance was mainly due to photogenerated holes of reactive species and TC was mainly degraded on the surface of the photocatalyst. Pathways of TC photodegradation were derived from the result of analyses of the reaction intermediates. In conclusion, Bi2Zr2O7 nanoparticles were found effective as photocatalyst for TC photodegradation by visible light.

Effect and mechanism analysis of MnO2 on permeable reactive barrier (PRB) system for the removal of tetracycline.

Effect of manganese dioxide (MnO2) on tetracycline (TC) removal/degradation in zero-valent iron (ZVI) based permeable reactive barrier (PRB) system was investigated. To analyze the role of MnO2, three different PRB columns packed with ZVI, ZVI and a layer of MnO2, and MnO2 were set up to investigate the removal effect and reaction mechanism of ZVI coupling with MnO2 on TC removal, respectively. The results show that the removal efficiencies of three PRB columns are 65%, 85%, and 50%, respectively. MnO2 could accelerate the transformation of Fe2+ into Fe3+ and combine with Fe3+ to degrade TC in different reaction sites in the ZVI-MnO2 PRB system. Hydroxyl radicals (·OH) were produced in this process, which contributed to about 58.3% for the TC degradation. The UV-Vis spectrum demonstrated that A ring of TC was the main reaction site for interaction with Fe3+ and the BCD rings were crucial for interactions with MnO2. On the basis of intermediates identified by LC-ESI-MS, the ring structure of TC was opened, and low-molecular-weight compounds were produced in ZVI-MnO2 PRB system.

Enhanced granulation and methane recovery at low load by downflow sludge circulation in anaerobic treatment of domestic wastewater.

The effects of downflow sludge circulation on granulation and methane recovery at low load were investigated for domestic wastewater treatment in a modified anaerobic reactor. Compared with conventional upflow anaerobic reactors, enhanced granulation with shortened start-up time was achieved and stable granules were successfully cultivated only after 58 days operations. The introduction of downflow sludge circulation resulted in reverse wastewater-sludge flow and uniform sludge distribution in the reaction zone, which contributed to enhanced wastewater-sludge mass transfer and satisfactory performance with a high soluble chemical oxygen demand (SCOD) removal efficiency of 94.8% at hydraulic retention time (HRT) 6 h. Besides, enhanced liquid-to-gas mass transfer caused a lower dissolved CH4 saturation index of 1.11 and a higher CH4 recovery efficiency of 79.48% at HRT 6 h. High throughput sequencing revealed a distinct shift of microbial community during start-up period from Proteobacteria to Bacteroidetes and Chloroflexi in the existence of downflow sludge circulation.