Responses of microbial communities to single-walled carbon nanotubes in phenol wastewater treatment systems

Boosting thermophilic anaerobic digestion with conductive materials: Current outlook and future prospects

Thermophilic anaerobic digestion (TAD) can provide superior process kinetics, higher methane yields, and more pathogen destruction than mesophilic anaerobic digestion (MAD). However, the broader application of TAD is still very limited, mainly due to process instabilities such as the accumulation of volatile fatty acids and ammonia inhibition in the digesters. An emerging technique to overcome the process disturbances in TAD and enhance the methane production rate is to add conductive materials (CMs) to the digester. Recent studies have revealed that CMs can promote direct interspecies electron transfer (DIET) among the microbial community, increasing the TAD performance. CMs exhibited a high potential for alleviating the accumulation of volatile fatty acids and inhibition caused by high ammonia levels. However, the types, properties, sources, and dosage of CMs can influence the process outcomes significantly, along with other process parameters such as the organic loading rates and the type of feedstocks. Therefore, it is imperative to critically review the recent research to understand the impacts of using different CMs in TAD. This review paper discusses the types and properties of CMs applied in TAD and the mechanisms of how they influence methanogenesis, digester start-up time, process disturbances, microbial community, and biogas desulfurization. The engineering challenges for industrial-scale applications and environmental risks were also discussed. Finally, critical research gaps have been identified to provide a framework for future research.

Recent advances in modification of novel carbon-based composites: Synthesis, properties, and biotechnological/ biomedical applications

Nowadays, carbon-based materials owing to great interest in biomedical science/biotechnology and applied for effective diagnosis and treatment of disease. To enhance the effectiveness of carbon nanotubes (CNTs)/graphene-based materials for bio-medical science/technology applications, different kinds of surface modification/functionalization were developed for the attachment of metal oxides nanostructures, biomolecules and polymers. The attachment of pharmaceutical agents with CNTs/graphene, make it a favorable candidate in research field of bio-medical science/technology applications. Surface modified/functionalized CNTs and graphene derivatives materials integrated with pharmaceutical agents has been developed for the purpose of cancer therapy, antibacterial action, pathogens bio detection, drug and gene delivery. Surface modification or functionalization of CNT/graphene materials provides good platform for pharmaceutical agents attachment with improved surface Raman scattering, fluorescence and its quenching capability. Graphene-based biosensing and bioimaging technologies are widely applied to identify numerous trace level analytes. These fluorescent and electrochemical sensors are utilized primarily for detecting organic, inorganic, and biomolecules. In this article, we highlights and summarized overview of the current research progress concerned on the CNTs/graphene-based materials as a new generation materials for detection and treatment of diseases.

The effects of g-C3N4/biochar and g-C3N4 on bacterial community in riverbed sediment

Biochar had been widely used to improve the activity of photocatalysts, the biochar-based photocatalysts had more potential for environmental pollution remediation, but their effect on the sediment remained unknown. To understand these, the typical photocatalyst g-C₃N₄ was modified by biochar to develop g-C₃N₄/biochar with enhanced photocatalytic ability. Riverbed sediment was exposed to g-C₃N₄ and g-C₃N₄/biochar respectively for 30 days, and Illumina sequencing was utilized to examine the changes in the bacterial community in the sediment. The results showed that in riverbed sediment, g-C₃N₄ exposure had a concentration-dependent effect on the diversity of bacteria, while g-C₃N₄/biochar exposure had a slight influence on the bacterial diversity and the diversity almost maintained stable with different g-C₃N₄/biochar concentration. The application of g-C₃N₄ exhibited an inhibition influence on the growth of Acidobacteria, Gemmatimonadetes, and Rokubacteria in sediment, whose relative abundance increased when g-C₃N₄ was 25 mg/kg, and then decreased when g-C₃N₄ beyond this concentration. The presence of g-C₃N₄/biochar increased the relative abundance of Cyanobacteria in sediment and showed no obvious impact on other dominant phyla. Both g-C₃N₄ and g-C₃N₄/biochar could alter the levels of TP, NN, and AN in the sediment, but the magnitude of the changes of these physicochemical factors caused by g-C₃N₄/biochar was much smaller than those caused by g-C₃N₄. In addition, the complexity of the bacterial community network was reduced in a high concentration of g-C₃N₄, while it remained stable with different concentrations of g-C₃N₄/biochar treatments. Totally, this study demonstrated that, compared to g-C₃N₄, g-C₃N₄/biochar was able to maintain the relative stability of the bacterial community in riverbed sediment and mitigate the negative effects of photocatalysts to some extent, making biochar an ecological remediation agent with great potential for application.

Effects of Carbonaceous Materials with Different Structures on Cadmium Fractions and Microecology in Cadmium-Contaminated Soils

Carbonaceous materials have proved to be effective in cadmium remediation, but their influences on soil microecology have not been studied well. Taking the structural differences and the maintenance of soil health as the entry point, we chose graphene (G), multi-walled carbon nanotubes (MWCNTs), and wetland plant-based biochar (ZBC) as natural and engineered carbonaceous materials to explore their effects on Cd fractions, nutrients, enzyme activities, and microbial communities in soils. The results showed that ZBC had stronger electronegativity and more oxygen-containing functional groups, which were related to its better performance in reducing soil acid-extractable cadmium (EX-Cd) among the three materials, with a reduction rate of 2.83-9.44%. Additionally, ZBC had greater positive effects in terms of improving soil properties, nutrients, and enzyme activities. Redundancy analysis and correlation analysis showed that ZBC could increase the content of organic matter and available potassium, enhance the activity of urease and sucrase, and regulate individual bacterial abundance, thereby reducing soil EX-Cd. Three carbonaceous materials could maintain the diversity of soil microorganisms and the stability of the microbial community structures to a certain extent, except for the high-dose application of ZBC. In conclusion, ZBC could better immobilize Cd and maintain soil health in a short period of time.

How multi-walled carbon nanotubes in wastewater influence the fate of coexisting antibiotic resistant genes in the subsequent disinfection process

Wastewater treatment plants (WWTPs) are important hubs for the spread of antibiotic resistance genes (ARGs). Engineered nanoparticles, which was inevitably released to WWTPs, could change environmentally sensitive of antibiotic resistant bacteria (ARB). This would influence the fate of ARGs in subsequent disinfection process and consequent health risk. In this study, the ARGs fate of the effluent in conventional sodium hypochlorite (NaClO) disinfection process was investigated as multi-walled carbon nanotubes (MWCNTs) existed in sequencing batch reactor (SBR). The results showed the existence of MWCNTs in SBR could enhance the removal efficiency of intracellular 16S rRNA gene and intI1, extracellular intI1, sul2 and tetX in the effluent by NaClO. This is mainly due to the variation of bacterial physiological status, bacterial population structure and the activation of NaClO under the role of MWCNTs. MWCNTs in SBR could increase in membrane permeability of bacterial cells, which would be conducive to the penetration of chlorination to cytoplasm. MWCNTs in SBR also could change the bacterial population structure and induce the chlorine-sensitive bacteria; thus the potential hosts of ARGs in the effluent would be more easily inactivated by NaClO. Moreover, the residual MWCNTs in the effluent could activate NaClO to generate various free radical, which would enhance the oxidizing capacity of chlorination.

Performance of bioelectrode based on different carbon materials in bioelectrochemical anaerobic digestion for methanation of maize straw

The performance of the bioelectrochemical anaerobic digestion (BEAD) reactor was investigated with different carbon material-modified electrodes for the methanation of maize straw. The carbon material-modified electrodes used titanium (Ti) mesh modified with carbon nanotube (CNT), carbon black (CB), and activated carbon (AC). The maximum cumulative methane production obtained in the Ti-CNT reactor was (616.4 ± 9.3) mL/g VS, while the maximum methane production rate in the Ti-AC reactor was (61.9 ± 1.0) mL/g VS.d.The electroactive bacteria were well enriched by the different electrodes, and the enriched electroactive bacteria further facilitate the direct interspecies electron transfer (DIET) for methane production. Additionally, we found the phylum Firmicutes showed a linear relationship to methanogenic performance, as well as the Genus Proteiniborus. The Ti-CNT electrode shows better performance by the electrochemical analysis. These findings provide critical knowledge for the large-scale use of the BEAD process and the treatment of maize straw.

Efficient degradation of naproxen in a three dimensional biofilm electrode magnetism reactor (3DBEMR): Removal performance and microbial community

A three-dimensional biofilm electrode magnetism reactor (3DBEMR) was constructed to removal naproxen (NPX). This study evaluated 3DBEMR performance in removal of refractory NPX, while also discussing the effect of the electro-magnetic superposition on microbial community by high throughput sequencing. Results indicated that 3DBEMR's average removal rate for NPX stood at 88.36%, representing an increase by 75.24%, 65.03% and 12.36%, respectively, compared to 3DBR (Three-Dimensional Biofilm Reactor), 3DBMR (Three-Dimensional Biofilm Magnetism Reactor) and 3DBER (Three-Dimensional Biofilm Electrode Reactor). This was attributed to the influence of electro-magnetic adsorption, electro-oxidaton/catalysis, and electro-magnetic biodegradation. Another major contributing factor to NPX removal was the presence in 3DBEMR of high-abundance genera such as Rhodobacter, Porphyrobacter, Methyloversatilis, Sphingopyxis,Bosea, Singulisphaera, Sphingomonas. Therefore, the 3DBEMR was successfully demonstrated to be a flexible and effective technique in NPX degradation, which would help to better understand the effect of superposition of electric and magnetic fields on microbial community.

Comparative effects of environmental factors on bacterial communities in two types of indoor dust: Potential risks to university students

University students are constantly exposed to potential bacterial pathogens and environmental pollutants in indoor environment because they spend most of their time indoors. University dormitory and printing shop are two typical indoor environments frequented by university students. However, little is known about the characteristics of bacterial community as well as the effect of indoor environmental factors on them. 16S rRNA gene sequencing was used to reveal the bacterial community in indoor dust, electronic devices were recorded during dust sampling, and polybrominated diphenyl ethers (PBDEs) were detected by gas chromatography mass spectrometry (GC-MS). Proteobacteria, Actinobacteria and Firmicutes were leading phyla, and Acinetobacter, Paracoccus and Kocuria were dominating genera. The predominant genera showed Acinetobacter > Paracoccus > unidentified Corynebacteriaceae in indoor dusts from university dormitories, whereas Paracoccus > unidentified Cyanobacteria > Acinetobacter in printing shops. The occurrence of Acinetobacter, Kocuria, Corynebacterium, Pseudomonas, and Bacillus suggested the health risks of potential pathogenic bacteria to university students. Significant differences of microbial composition and diversity were proved between university dormitories and printing shops. Chemoheterotrophy and aerobic chemoheterotrophy were dominant bacterial functions, and the seven primary bacterial functions displayed university dormitory > printing shop. BDE 138 and BDE 66 were main environmental parameters affecting the indoor dust bacterial community in university dormitory, while printer and BDE 47 played dominating role in shaping microorganism in printing shop. The complex biotic (potential bacterial pathogens) and abiotic factors (electronic equipment and chemical pollutants) in indoor dusts may pose potential health risks to university students.

Insight investigation of the on-site activated sludge reduction induced by metabolic uncoupler: Effects of 2,6-dichlorophenol on soluble microbial products, microbial activity, and environmental impact

Metabolic uncoupling technology was one of the methods widely used to on-site control the production of excess sludge in wastewater treatment processes. However, the uncoupler effects on soluble microbial products (SMP), microbial activity, and environment impact have few been reported. This study showed that sludge yield was reduced by 33.3% at 2,6-dichlorophenol (2,6-DCP) concentrations of 10 mg/L. The addition of 2,6-DCP also reduced the content of polysaccharide and protein in SMP, and the three-dimension excitation emission matrix (3D-EEM) suggested that the fluorescence intensities of humic acid-like, fulvic acid-like, and tryptophan protein-like substances decreased, proving that 2,6-DCP addition will weaken the interaction between microorganisms and the environmental matrix. Moreover, 2,6-DCP addition will change the microbial morphology and community of activated sludge. The active or respiring bacteria portion was lessened, and sludge flocs become dispersed, but it will not affect its settling performance. Surprisingly, 2,6-DCP has certain biodegradability and could be used as an environmentally friendly metabolic uncoupler under low-concentration dosing conditions. This study systematically evaluated the effect of 2,6-DCP on sludge production, SMP contents, microbial morphology, microbial community, demonstrating the environmental impact and application feasibility in the wastewater treatment systems.

Effects of sludge thermal hydrolysis pretreatment on anaerobic digestion and downstream processes: mechanism, challenges and solutions

Thermal hydrolysis pretreatment (THP), as a step prior to sludge anaerobic digestion (AD), is widely applied due to its effectiveness in enhancing organic solids hydrolysis and subsequent biogas productivity. However, THP also induces a series of problems including formation of refractory compounds in THP cylinder, high residual ammonia and organic in the AD centrate, inhibition on downstream nitrogen removal process and reduction in UV-disinfection effectiveness during post-treatment. More attention should be paid on how to mitigate these negative effects. Despite intensive studies were carried out to reduce refractory compounds formation and enhance biological performance, there is limited effort to discuss the solutions to tackle the THP associated problems in a holistic manner. This paper summarizes the solutions developed to date and analyzes their technology readiness to assess application potential in full-scale settings. The content highlights the limitations of THP and proposes potential solutions to address the technological challenges.

Wastewater treatment nexus: Carbon nanomaterials towards potential aquatic ecotoxicity

Carbon nanomaterials (CNMs) provide an effective solution and a novel advancement for wastewater treatment. In this review, a total of 3823 bibliographic records derived from recent 10 years are visualized based on scientometric analysis. The results indicate metal-free CNMs-mediated advanced oxidation processes (AOPs) might be a motive force to develop CNMs application for wastewater treatment; however, corresponding evaluations of aquatic toxicity still lack sufficient attention. Therefore, recent breakthroughs and topical innovations related to prevalent wastewater treatment technologies (i.e., adsorption, catalysis and membrane separation) using three typical dimensional CNMs (nanodiamonds, carbon nanotubes, and graphene-based nanomaterials) are comprehensively summarized in-depth, along with a compendious introduction to some novel techniques (e.g., computational simulation) for identifying reaction mechanisms. Then, current research focusing on CNMs-associated aquatic toxicity is discussed thoroughly, mainly demonstrating: (1) the adverse effects on aquatic organisms should not be overlooked prior to large-scale CNMs application; (2) divergent consequences can be further reduced if the ecological niche of aquatic organisms is emphasized; and (3) further investigations on joint toxicity can provide greater beneficial insight into realistic exposure scenarios. Finally, ongoing challenges and developmental directions of CNMs-based wastewater treatment and evaluation of its aquatic toxicity are pinpointed and shaped in terms of future research.

Zeolitic imidazolate framework-derived porous carbon enhances methanogenesis by facilitating interspecies electron transfer: Understanding fluorimetric and electrochemical responses of multi-layered extracellular polymeric substances

Modulating microbial electron transfer during anaerobic digestion can significantly improve syntrophic interactions for enhanced biogas production. As a carbonaceous conductive material, zeolite imidazolate framework-67 (ZIF-67)-derived porous carbon (PC) was hypothesized to act as a microbial electron transfer highway and assessed with respect to understanding the fluorimetric and electrochemical responses of multilayered extracellular polymeric substances (EPS). The highest biomethane yield (614.0 mL/g) from ethanol was achieved in the presence of 100 mg/L PC prepared at a carbonization temperature of 800 °C (PC-800), which was 28.2% higher than that without PC addition. Electrochemical analysis revealed that both the redox peak currents and conductivity of the methanogenic sludge increased, while the free charge transfer resistance decreased with PC-800 addition. The conductive PC-800 potentially functioned as an abiotic electron conduit to promote direct interspecies electron transfer, thereby resulting in decreased expression of functional genes associated with electrically conductive pili (e-pili) and hemeproteins. Additionally, PC-800 stimulated the secretion of redox-active humic substances (HSs), and excitation emission matrix spectra analysis indicated that the largest increase in percent fluorescence response of HSs occurred in the tightly bound EPS (TB-EPS) with addition of PC-800. This was attributed to the strong complexation ability of PC-800 particles to hydroxyl/carboxylic/phenolic moieties of HSs contained in the TB-EPS. Microbial analysis revealed that syntrophic/exoelectrogenic bacteria such as Pelotomaculum and Syntrophomonas, as well as hydrogenotrophic/electrotrophic methanogens such as Methanoculleus and Methanobacterium, were enriched in methanogenic sludge with adding PC-800. This study provided comprehensive insights for understanding the interactions among ZIF-derived PC, methanogenic microorganisms and their multilayered EPS.

Highly efficient and selectivity removal of heavy metal ions using single-layer NaxKyMnO2 nanosheet: A combination of experimental and theoretical study

Manganese oxides (MnO₂) are widely applied in heavy metal ions removal due to their low-cost, environmental-friendly and biocompatibility. However, the adsorption capacity of MnO₂ need to be further improved to satisfy the demand of practical application. Herein, a highly dispersed single layer Na_xK_yMnO₂ nanosheet was synthesized by a facile wet-chemical method with sodium dodecyl sulfonate as surfactant. The high surface specific area, excellent dispersibility and abundant oxygen vacancies endowed Na_xK_yMnO₂ nanosheets with potential in heavy metal ions adsorption. The adsorption experiments results showed that Na_xK_yMnO₂ nanosheets possessed high efficiency and selectivity towards lead ion (Pb²⁺) with a high adsorption capacity of 2091.8 μmol g^-1. The Na_xK_yMnO₂ also showed an excellent reusability with the removal rate of 95.4% for Pb²⁺ even after five cycles. Moreover, both the theoretical calculation and experimental data illustrated that the single layer Na_xK_yMnO₂ nanosheets possess high selectivity to Pb²⁺ adsorption.

Stronger impacts of long-term relative to short-term exposure to carbon nanomaterials on soil bacterial communities

Environmental impacts of carbon nanomaterials (CNMs) have been attracting increasing concerns in recent years. Knowledge on how short-term exposure to CNMs influences soil microbial communities is available. However, little is known about the possible difference in effects of long-term versus short-term exposure of CNMs on soil microbial communities. In this study, we systematically compared effects of fullerene (C₆₀), single-walled carbon nanotubes (SW), and graphene (GR) on soil bacterial communities over short (30 d) and long (360 d) term exposure durations. Our findings revealed that short-term exposure to all CNMs significantly increased the alpha diversity of soil bacterial communities. SW and GR exposure for 360 d relative to that for 30 d more significantly decreased their alpha diversity. Compared to short-term exposure, a long term exposure to CNMs more strongly altered the beta diversity of soil bacterial communities. LEfSe analysis showed that, GR relative to C₆₀ and SW exposure more strongly altered soil bacterial community composition especially for long-term duration at various taxonomic levels; more taxa were also identified by LEfSe analysis as biomarkers upon long-term GR exposure. More OTUs were affected by long-term GR exposure. These differences resulted from both distinct physicochemical properties of various CNMs and their exposure durations.

Carbon-based conductive materials facilitated anaerobic co-digestion of agro waste under thermophilic conditions

Management of agro-waste is a major challenge globally due to inefficient disposal techniques, which concominantly leads pollution and loss of renewable bioenergy. Anaerobic digestion of agro-waste is one of the ways to tackle this problem but hindered by the recalcitrant nature of agro-waste. This study investigated the effect of granular activated carbon (GAC) and granular biochar (GBC) addition to enhance the thermophilic anaerobic co-digestion of wheat husk and sewage sludge. The conductive materials (particle size: 2-5 mm) were added separately at five different concentrations: 10, 20, 30, 40, 50 g/L_inoculum. The findings revealed that samples amended with GAC and GBC at 20 g/L dosage had the highest biogas yield of 263 and 273 mL/gVS_added, respectively, corresponding to 22 and 27% higher yield than the control. Additionally, a shorter lag phase was observed in both cases compared to the Control. However, the GBC amended samples showed relatively stable biogas production compared to GAC and consistent results regarding pH, alkalinity, total volatile fatty acids, and soluble chemical oxygen demand. The preliminary techno-economic analysis indicates that addition of GAC or GBC may not be feasible and require other innovative engineered solutions for the addition of conductive materials. This study confirms that GAC and GBC amendments enhance the biogas productivity and process stability in anaerobic digestion of recalcitrant agro-waste under the high-temperature regime and calls for further research in this direction.

Maintaining microbial diversity mitigates membrane fouling of an anoxic/oxic membrane bioreactor under starvation condition

The aim of this study was to evaluate the contribution of dissolved organic carbon (DOC) and microbial community dynamics to membrane fouling development in membrane bioreactor (MBR). We operated laboratory-scale anoxic/oxic-MBRs under prolonged starvation conditions in different seasons and the dynamics and diversity of the microbial communities were investigated. Although fouled-MBRs showed DOC accumulation in the activated sludge (AS), the fouling-mitigated MBR suggested that dissolved oxygen was consumed and DOC of the sludge supernatant was degraded. 16S rRNA genes analysis of AS in the MBRs revealed that Chitinophagaceae and Candidatus Promineofilum specifically increased in the fouling-mitigated MBR, suggesting that they played important roles in membrane fouling mitigation; high microbial diversity in the reactor also contributed to fouling mitigation. In the fouled reactor, enrichment of Xanthomonadaceae might be related to fouling causing substances formation leading to membrane fouling development; lower microbial diversity also contributed to fouling development in the fouled MBR.

Carbon Nanotubes-Based Nanomaterials and Their Agricultural and Biotechnological Applications

Carbon nanotubes (CNTs) are considered a promising nanomaterial for diverse applications owing to their attractive physicochemical properties such as high surface area, superior mechanical and thermal strength, electrochemical activity, and so on. Different techniques like arc discharge, laser vaporization, chemical vapor deposition (CVD), and vapor phase growth are explored for the synthesis of CNTs. Each technique has advantages and disadvantages. The physicochemical properties of the synthesized CNTs are profoundly affected by the techniques used in the synthesis process. Here, we briefly described the standard methods applied in the synthesis of CNTs and their use in the agricultural and biotechnological fields. Notably, better seed germination or plant growth was noted in the presence of CNTs than the control. However, the exact mechanism of action is still unclear. Significant improvements in the electrochemical performances have been observed in CNTs-doped electrodes than those of pure. CNTs or their derivatives are also utilized in wastewater treatment. The high surface area and the presence of different functional groups in the functionalized CNTs facilitate the better adsorption of toxic metal ions or other chemical moieties. CNTs or their derivatives can be applied for the storage of hydrogen as an energy source. It has been observed that the temperature widely influences the hydrogen storage ability of CNTs. This review paper highlighted some recent development on electrochemical platforms over single-walled CNTs (SWCNTs), multi-walled CNTs (MWCNTs), and nanocomposites as a promising biomaterial in the field of agriculture and biotechnology. It is possible to tune the properties of carbon-based nanomaterials by functionalization of their structure to use as an engineering toolkit for different applications, including agricultural and biotechnological fields.

Co-metabolism for enhanced phenol degradation and bioelectricity generation in microbial fuel cell

Co-metabolism is one of the effective approaches to increase the removal of refractory pollutants in microbial fuel cells (MFCs), but studies on the links between the co-substrates and biodegradation remain limited. In this study, four external carbon resources were used as co-substrates for phenol removal and power generation in MFC. The result demonstrated that acetate was the most efficient co-substrate with an initial phenol degradation of 78.8% and the voltage output of 389.0 mV. Polarization curves and cyclic voltammogram analysis indicated that acetate significantly increased the activity of extracellular electron transfer (EET) enzyme of the anodic microorganism, such as cytochrome c OmcA. GC-MS and LC-MS results suggested that phenol was biodegraded via catechol, 2-hydroxymuconic semialdehyde, and pyruvic acid, and these intermediates were reduced apparently in acetate feeding MFC. The microbial community analysis by high-throughput sequencing showed that Acidovorax, Geobacter, and Thauera were predominant species when using acetate as co-substrate. It can be concluded that the efficient removal of phenol was contributed to the positive interactions between electrochemically active bacteria and phenolic degradation bacteria. This study might provide new insight into the positive role of the co-substrate during the treatment of phenolic wastewater by MFC.

Characterization of sludge reduction and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time

Sludge reduction performance and bacterial community dynamics in a pilot-scale multi-stage digester system with prolonged sludge retention time were characterized. Throughout the operation period of 281 days, the total loading sludge and the total digested sludge were 4700 and 3300 kg-MLSS. After 114 days of operation, the residual MLSS (RMLSS) in the reactors for sludge treatment was maintained at 18-25 kg-RMLSS m^-3, and the sludge reduction efficiency achieved 95% under the F/M ratio (kg-loading MLSS kg-RMLSS^-1) of less than 0.018. Also, among the sludge components, both fixed suspended solids and volatile suspended solids were reduced. Based on the sludge reduction performance and the RNA-based bacterial community characteristics, the combined action of the maintenance metabolism, lysis-cryptic growth, and particulate inorganic matter is proposed as the sludge reduction mechanism in the multi-stage sludge treatment process.

Formation of microbial products by activated sludge in the presence of a metabolic uncoupler o-chlorophenol in long-term operated sequencing batch reactors

Metabolic uncouplers are widely used for reducing excess sludge in biological wastewater treatment systems. However, the formation of microbial products, such as extracellular polymeric substances, polyhydroxyalkanoate and soluble microbial products by activated sludge in the presence of metabolic uncouplers remains unrevealed. In this study, the impacts of a metabolic uncoupler o-chlorophenol (oCP) on the reduction of activated sludge yield and formation of microbial products in laboratory-scale sequencing batch reactors (SBRs) were evaluated for a long-term operation. The results show the average reduction of sludge yield in the four reactors was 17.40%, 25.80%, 33.02% and 39.50%, respectively, when dosing 5, 10, 15, and 20 mg/L oCP. The oCP addition slightly reduced the pollutant removal efficiency and decreased the formation of soluble microbial products in the SBRs, but stimulated the productions of extracellular polymeric substances and polyhydroxyalkanoate in activated sludge. Furthermore, the significant reduction of electronic transport system activity occurred after the oCP addition. Microbial community analysis of the activated sludge indicates dosing oCP resulted in a decrease of sludge richness and diversity in the SBRs. Hopefully, this study would provide useful information for reducing sludge yield in biological wastewater treatment systems and behaviors of activated sludge in the presence of uncouplers.

Effects of carbon nanotubes on biodegradation of pollutants: Positive or negative?

Recently, a large quantity of carbon nanotubes (CNTs) enters the environment due to the increasing production and applications. More and more researches are focused on the fate and possible ecological risks of CNTs. Some literatures summarized the effects of CNTs on the chemical behavior and fate of pollutants. However, little reviewed the effects of CNTs on the biodegradation of pollutants. In general, the effects of CNTs on the biodegradation of pollutants and the related mechanisms were summarized in this review. CNTs have positive or negative effects on the biodegradation of contaminants by affecting the functional microorganisms, enzymes and the bioavailability of pollutants. CNTs may affect the microbial growth, activity, biomass, community composition, diversity and the activity of enzymes. The decrease of the bioavailability of pollutants due to the sorption on CNTs also causes the reduction of the biodegradation of contaminants. In addition, the roles of CNTs are controlled by multiple mechanisms, which are divided into three aspects i.e., properties of CNTs, environment condition, and microorganisms themself. The better understanding of the fate of CNTs and their impacts on the biochemical process in the environment is conducive to determine the release of CNTs into the environment.

Mutual effect between electrochemically active bacteria (EAB) and azo dye in bio-electrochemical system (BES)

Herein, the mutual effect between azo dye and the performance of electrochemically active bacteria (EAB) is investigated in detail, which is crucial to understand and control the bio-electrochemical systems (BESs) operation for azo dye containing wastewater treatment. EAB is enriched at controlled potential of -0.2 V vs Ag/AgCl in single-chamber BESs. Over 95% azo dye (alizarin yellow R (AYR)) was decolorized regardless of the initial AYR concentration ranging from 30 to 120 mg/L within 24 h. The fastest decolorization rate was obtained at AYR initial concentration of 70 mg/L, which was 4.25 times greater in the closed circuit BESs than that in the open circuit one. 16S rRNA gene based microbial community analysis showed that Geobacter was dominant in EAB with relative abundance increased from 77.98% (0 mg/L AYR) to 92.22% (70 mg/L AYR), indicating that azo dye selectively boosts the growth of exoelectrogens in electrode biofilm communities. Under electricity stimulation, extracellular process can be mutually conducted by azo dye compounds, which is favorable for accelerating reaction rate and avoiding of significant toxic effect on EAB.

A comprehensive comparison between non-bulking and bulking aerobic granular sludge in microbial communities

Filamentous sludge bulking poses great threats to operational stability of aerobic granular sludge. Exploration of the microbial community aids knowledge of the causative factors to sludge bulking and guides directions for corresponding actions for prevention and controlling. Detailed changes of bacterial community within the non-bulking and bulking were performed and compared with a non-specific method through 1‰ (v/v) hydrogen peroxide (H₂O₂) addition. Results revealed that non-bulking/bulking granules maintained effective carbon and nitrogen removal, while bulking completely deteriorated enhanced biological phosphorus removal (EBPR). Excess extracellular polymeric substances (EPS) especially polysaccharide (PS) were directly linked with sludge bulking and abundant PS contributed to subsequent granular re-stability. Filamentous bulking dramatically altered the bacterial populations and 1‰ H₂O₂ effectively controlled bulking by eliminating causative filaments Singulisphaera and Thiothrix. Together, this study provides new insights into the non-bulking/bulking granules and could direct the prevention and control of filamentous bulking in aerobic granules.

Insights into long-term effects of amino-functionalized multi-walled carbon nanotubes (MWCNTs-NH2) on the performance, enzymatic activity and microbial community of sequencing batch reactor

Carbon nanotubes (CNTs) inevitably enter domestic sewage and industrial wastewater with the continuous increase of their production and application field. The potential effect of CNTs on biological wastewater treatment processes has raised wide concerns due to their biotoxicity. In the present study, the performance, microbial community and enzymatic activity of sequencing batch reactors (SBRs) were evaluated under 148-day exposure of amino-functionalized multi-walled CNTs (MWCNTs-NH₂) at 10 and 30 mg/L. The COD removal efficiency at 10 and 30 mg/L MWCNTs-NH₂ gradually reduced from 91.03% and 90.43% on day to 89.11% and 86.70% on day 148, respectively. The NH₄⁺-N removal efficiency at 10 and 30 mg/L MWCNTs-NH₂ gradually reduced from 98.98% and 98.46% on day 1 to 96.65% and 63.39% on day 148, respectively. Compared to 0 mg/L MWCNTs-NH₂, the oxygen-utilizing rate, ammonia-oxidizing rate, nitrite-oxidizing rate, nitrite-reducing rate and nitrate-reducing rate at 30 mg/L MWCNTs-NH₂ were decreased by 52.35%, 60.58%, 55.12%, 56.56% and 57.42% on day 148, respectively. The microbial reactive oxygen species and lactate dehydrogenase release on day 148 was increased by 59.71% and 55.28% at 30 mg/L MWCNTs-NH₂, respectively. The key microbial enzymatic activity related to nitrogen removal decreased with the increase of operation time under MWCNTs-NH₂ stress. The relative abundances of Nitrosomonas, Nitrosospira, Nitrospira and some denitrifying bacteria at 10 mg/L MWCNTs-NH₂ gradually reduced with an increment in operation time. The changes of nitrogen removal rate, microbial community and enzymatic activity of SBR were related to the time-cumulative nonlinear inhibition effect under long-term exposure.

Simultaneous nitrification and denitrification (SND) bioaugmentation with Pseudomonas sp. HJ3 inoculated for enhancing phenol and nitrogen removal in coal gasification wastewater

A simultaneous nitrification and denitrification (SND) bioaugmention system with Pseudomonas sp. HJ3 inoculated was established to explore the potential of simultaneous phenol and nitrogen removal in coal gasification wastewater (CGW). When the concentration of influent chemical oxygen demand (COD) and total phenols (TPh) was 1,765.94 ± 27.43 mg/L and 289.55 ± 10.32 mg/L, the average removal efficiency of COD and TPh at the stable operating stage reached 64.07% ± 0.76% and 74.91% ± 0.33%, respectively. Meanwhile, the average removal efficiency of NH₄ ⁺-N and total nitrogen (TN) reached 67.96% ± 0.17% and 57.95% ± 0.12%, respectively. The maximum SND efficiency reached 83.51%. Furthermore, SND bioaugmentation performed with good nitrification tolerance of phenol shock load and significantly reduced toxic inhibition of organisms. Additionally, the microbial community analysis indicated that Pseudomonas sp. HJ3 was the predominant bacterium in the SND bioaugmentation system. Moreover, the indigenous nitrogen removal bacteria such as Thauera, Acidovorax and Stenotrophomonas were enriched, which further enhanced the nitrogen removal in the SND bioaugmentation system. The results demonstrated the promising application of SND bioaugmentation for enhancing simultaneous phenol and nitrogen removal in CGW treatment.

Effects of biochar on the phenol treatment performance and microbial communities shift in sequencing batch reactors

The extensive application of biochar (BC) attracts concerns regarding its environmental effect. Wastewater treatment systems are potential BC recipients; however, the impacts of BC on these systems are still unclear. In this study, effects of BC on the phenol treatment performance and shift in microbial communities in sequencing batch reactor (SBR) were investigated. The phenol degradation rates were enhanced in all BC-treated SBRs during the whole operation due to promotion of key enzymes involved in phenol degradation. The decrease in abundance of intracellular reactive oxygen species (ROS) in SBRs indicated that BC protected microorganisms by ameliorating phenol toxicity, leading to a decrease in the secretion of extracellular polymeric substances (EPS). The functional groups, protein (C=O, -CO-NH), polysaccharide (C-OH, C-O-C, C-O), and nucleic acids (O-P-O) associated bonds of EPS decreased or disappeared in BC-treated SBRs. Miseq sequencing revealed significant decrease in bacterial diversity and remarkable changes in the bacterial community structure in BC-treated SBRs. Abundances of Comamonas and Cupriavidus increased significantly upon BC exposure, which contributed to phenol degradation. Treatment with relatively high BC dosage exhibited considerable inhibition on Thauera. Canonical correspondence analysis (CCA) indicated that the shift in abundances of these genera was closely associated with BC dosage. This study suggested that BC exerted protective effects on sludge microbes of phenol wastewater treatment systems, while it affected the bacterial community structure and diversity at test concentrations. Thus, this study elucidates the comprehensive effects of BC on wastewater treatment process.

Long-term impacts of carboxyl functionalized multi-walled carbon nanotubes on the performance, microbial enzymatic activity and microbial community of sequencing batch reactor

The performance, microbial community and enzymatic activity of sequencing batch reactors (SBRs) were evaluated under long-term exposure of 0, 10 and 30 mg/L carboxyl functionalized multi-walled carbon nanotubes (MWCNTs-COOH). The presence of 10 mg/L MWCNTs-COOH displayed no adverse impacts on the COD and NH₄⁺-N removal of SBR, whereas 30 mg/L MWCNTs-COOH declined the COD and NH₄⁺-N removal. MWCNTs-COOH inhibited the denitrifying process and led to the accumulation of effluent NO₂^--N concentration. The inhibition of MWCNTs-COOH on the oxygen utilization rate, nitrogen removal rate and enzymatic activity of activated sludge gradually enhanced with the increase of operating time and influent MWCNTs-COOH concentration. MWCNTs-COOH stimulated more reactive oxygen species production and lactate dehydrogenase release, which might affect the microbial physiological functions and morphology. The microbial diversity and richness was declined evidently after long-term exposure of MWCNTs-COOH. The relative abundance of nitrifying and denitrifying bacteria showed some changes under MWCNTs-COOH stress.

Antibacterial activity of chitosan nano-composites and carbon nanotubes: A review

Bacteriological contamination of water sources is a major challenge that has a detrimental impact on both the environment and human health. This imposes the search for the most efficient disinfectant. Despite their antibacterial efficiency, traditional methods can often form disinfection byproducts through their reaction with organic and inorganic compounds. Substitutes for conventional bacterial inactivation methods should not produce harmful byproducts and must also be cost effective. Nanotechnology is an attractive option that is suited for surface reactions as nanostructures offer large surface to volume ratios. Technologies using chitosan-modified nanocomposites and carbon nanotubes have proven to offer promising alternatives for bacterial inactivation. To enhance their antibacterial efficiency, such technologies have been modified chemically and physically and have as well been associated with other treatment techniques. However, despite their high bacterial disinfection efficacy and lack of treatment byproducts, the vagueness in bacterial inactivation mechanisms and complexity in materials preparation have often obscured their wide scale application. The aim of this manuscript is to review the recent advances in bacterial disinfection using nanomaterials, in the form of chitosan and carbon nanotubes. The rapid rate of research and the notable progress in this area dictate the frequent compilation and dissemination of recent introductions to this field. Existing gaps in the literature are thus also highlighted and reported discrepancies are pinpointed so that roadmaps for future studies may be figured.

Effects of Various Carbon Nanotubes on Soil Bacterial Community Composition and Structure

Carbon nanotubes (CNTs) have huge industrial potential, and their environmental impacts need to be evaluated. Knowledge of CNT impacts on soil microbial communities is still limited. To address this knowledge gap, we systematically examined dynamic effects of one type of single-walled carbon nanotubes (SWs) and three multiwalled carbon nanotubes (MWs) with different outer diameters on the soil bacterial community in an agricultural soil over 56 days. The results showed that SWs differently affected soil bacterial abundance, diversity, and composition as compared to MWs. The differences could have resulted from the materials' distinct physical structure and surface composition, which in turn affected their bioavailability in soil. For certain treatments, soil bacterial diversity and the relative abundance of certain predominant phyla were correlated with their exposure duration. However, many phyla recovered to their initial relative abundance within 56 days, reflecting resilience of the soil bacterial community in response to CNT-induced disturbance. Further analysis at the genus level showed differential tolerance to MWs, as well as size- and dose-dependent tolerance among bacterial genera. Predictive functional profiling showed that while CNTs initially caused fluctuations in microbial community function, community function largely converged across all treatments by the end of the 56 day exposure.

Understanding the influence of carbon nanomaterials on microbial communities

Carbon nanomaterials (CNMs) are widely used because of their unique advantages in recent years. At the same time, the influence of CNMs on the environment is becoming increasingly prominent. This review mainly introduces the research progress in the effects of fullerenes, multi-walled carbon nanotubes (MWCNTs), single-walled carbon nanotubes (SWCNTs) and graphene on microorganisms and their toxicity mechanisms. On this basis, we have analyzed beneficial and adverse effects of fullerenes, graphene, MWCNTs and SWCNTs to microorganisms, and discussed the similarities of the toxicity mechanisms of different CNMs on microorganisms. This review helps provide ideas on how to protect microorganisms from the impacts of carbon nanomaterials, and it will be conductive to providing a strong theoretical basis for better application of carbon nanomaterials.

Deciphering of microbial community and antibiotic resistance genes in activated sludge reactors under high selective pressure of different antibiotics

Currently, the effects of high antibiotic concentrations on the performance of microbiota and antibiotic resistance genes (ARGs) in activated sludge (AS) process are not well characterized. Lab-scale batch reactors were performed to evaluate the dynamics of microbial community and ARGs in response to six antibiotics at different concentrations using high-throughput sequencing-based 16S rRNA gene and metagenomic analyses. The presence of antibiotics remarkably decreased the microbial diversity, caused a great change of the microbiota structure, and exerted a selective pressure on the enrichment of potential antibiotic resistant bacteria (ARB), such as Arthrobacter, Thauera, Geothrix, Rudaea, Aridibacter, Conexibacter, Terrimonas, etc. High antibiotic selective pressures increased ARG abundance but simultaneously reduced ARG number. In total, 491 ARG subtypes belonging to 20 ARG types were detected and kanamycin treatment showed the highest ARG abundances. A core set of 54 ARG subtypes that accounted for 66.7%-99.6% of the total ARG abundances were shared by all samples. The increase of the abundances of both corresponding and non-corresponding ARGs under a specific antibiotic treatment revealed the collateral effects of antibiotic selective pressure. Microbial community may play an important role in the composition of ARGs. Network analysis indicated that both internal-type and external-type of ARGs exhibited higher non-random co-occurrence incidences and 18 genera were speculated as the possible hosts for multiple ARGs. This study deciphered the profiles and relationships between microbial community and ARGs in AS process treating wastewater with high antibiotic concentrations and could provide helpful guidance for controlling the development and dissemination of ARB and ARGs.

Bioremediation of contaminated urban river sediment with methanol stimulation: Metabolic processes accompanied with microbial community changes

The intense pollution of urban river sediments with rapid urbanization has attracted considerable attention. Complex contaminated sediments urgently need to be remediated to conserve the ecological functions of impacted rivers. This study investigated the effect of using methanol as a co-substrate on the stimulation of the indigenous microbial consortium to enhance the bioremediation of petroleum hydrocarbons (PHs) and polycyclic aromatic hydrocarbons (PAHs) in an urban river sediment. After 65 days of treatment, the PAHs degradation efficiencies in the sediment adding methanol were 4.87%-40.3% higher than the control. The removal rate constant of C31 was 0.0749 d^-1 with 100 mM of supplied methanol, while the corresponding rate was 0.0399 d^-1 in the control. Four-ring PAHs were effectively removed at a degradation efficiency of 65%-69.8%, increased by 43.3% compared with the control. Sulfate reduction and methanogenesis activity were detected, and methane-producing archaea (such as Methanomethylovorans, with a relative abundance of 25.87%-58.53%) and the sulfate-reducing bacteria (SRB, such as Desulfobulbus and Desulfobacca) were enriched. In addition, the chemolithoautotrophic sulfur-oxidizing bacteria (SOB, such as Sulfuricurvum, with a relative abundance of 34%-39.2%) were predominant after the depletion of total organic carbon (TOC), and markedly positively correlated with the PHs and PAHs degradation efficiencies (P < 0.01). The SRB and SOB populations participated in the sulfur cycle, which was associated with PHs and PAHs degradation. Other potential functional bacteria (such as Dechloromonas) were also obviously enriched and significantly positively correlated with the TOC concentration after methanol injection (P < 0.001). This study provides a new insight into the succession of the indigenous microbial community with methanol as a co-substrate for the enhanced bioremediation of complexly contaminated urban river sediments.

Response of chloramphenicol-reducing biocathode resistome to continuous electrical stimulation

Understanding the fate of overall antibiotic resistance genes (ARGs) during the biological treatment of antibiotic containing wastewater is a central issue for the water ecological safety assessment. Although the microbial electrode-respiration based biotransformation process could significantly detoxify some antibiotic contaminants, e.g. chloramphenicol (CAP), the response of CAP-reducing biocathode microbiome and resistome to continuous electrical stimulation, especially ARGs network interactions, are poorly understood. Here, using highthroughput functional gene array (GeoChip v4.6) and Illumina 16S rRNA gene MiSeq sequencing, the structure, composition, diversity and network interactions of CAP-reducing biocathode microbiome and resistome in response to continuous electrical stimulation were investigated. Our results indicate that the CAP bioelectroreduction process could significantly accelerate the elimination of antibacterial activity of CAP during CAP-containing wastewater treatment compared to the pure bioreduction process. Continuous electrical stimulation could obviously alter both the microbiome and resistome structures and consistently decrease the phylogenetic, functional and overall ARGs diversity and network complexity within the CAP-reducing biofilms. The relative abundances of overall ARGs and specific CAP resistance related major facilitator superfamily (MFS) transporter genes were significantly negatively correlated with the reduction efficiency of CAP to inactive antibacterial product AMCl (partially dechlorinated aromatic amine), which may reduce the ecological risk associated with the evolution of multidrug-resistant bacteria and ARGs during antibiotic-containing wastewater treatment process. This study offers new insights into the response of an antibiotic reducing biocathode resistome to continuous electrical stimulation and provides useful information on the assessment of overall ARGs risk for the bioelectrochemical treatment of antibiotic contaminants.

The mechanism of synergistic effect between iron-carbon microelectrolysis and biodegradation for strengthening phenols removal in coal gasification wastewater treatment

A novel iron-carbon microelectrolysis (ICME) inoculated with activated sludge (AS) process was specifically designed to look into the roles of microelectrolysis and biodegradation as well as their synergistic effect on phenols removal in coal gasification wastewater (CGW) treatment. The results indicated that the removal efficiency of COD, phenols and TOC in integrated ICME-AS process reached 87.36 ± 2.98%, 92.62 ± 0.76% and 84.45 ± 0.65%, respectively. Moreover, phenols-degrading bacteria and electrochemical-active bacteria presented better adaptability to phenolic impact. Meanwhile their syntrophic interaction was driven under the simulation of microelectrolysis. Furthermore, electrochemical redox efficiency was significantly improved, and the corresponding maximum power output reached 0.043 ± 0.01 mW/cm². Apparently, the synergistic effect between microelectrolysis and biological action effectively strengthened phenols degradation and electricity generation. The results proved that the integrated ICME-AS process was a promising technology applied for CGW and other refractory industrial wastewater treatments.

Graphene Facilitates Biomethane Production from Protein-Derived Glycine in Anaerobic Digestion

Interspecies electron transfer is a fundamental factor determining the efficiency of anaerobic digestion (AD), which involves syntrophy between fermentative bacteria and methanogens. Direct interspecies electron transfer (DIET) induced by conductive materials can optimize this process offering a significant improvement over indirect electron transfer. Herein, conductive graphene was applied in the AD of protein-derived glycine to establish DIET. The electron-producing reaction via DIET is thermodynamically more favorable and exhibits a more negative Gibbs free energy value (−60.0 kJ/mol) than indirect hydrogen transfer (−33.4 kJ/mol). The Gompertz model indicated that the kinetic parameters exhibited linear correlations with graphene addition from 0.25 to 1.0 g/L, leading to the highest increase in peak biomethane production rate of 28%. Sedimentibacter (7.8% in abundance) and archaea Methanobacterium (71.1%) and Methanosarcina (11.3%) might be responsible for DIET. This research can open up DIET to a range of protein-rich substrates, such as algae.

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Graphene led to an increase in peak bio-CH₄ production rate from glycine by 28%

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Kinetic parameters had linear correlations with graphene addition (0.25–1.0 g/L)

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Direct interspecies electron transfer (DIET) contributed to the improved performance

New insights into enhanced anaerobic degradation of coal gasification wastewater (CGW) with the assistance of graphene

The up-flow anaerobic sludge blanket (UASB) system with graphene assisted was developed for coal gasification wastewater (CGW) treatment. Short-term results showed that optimal graphene addition (0.5 g/L) resulted in a more significant enhancement of methane production and chemical oxygen demand (COD) removal compared with that of the optimal activated carbon addition (10.0 g/L). Long-term results demonstrated that COD removal efficiency and methane production rate with graphene assisted achieved 64.7% and 180.5 mL/d, respectively. In addition, graphene could promote microbes accumulation and enzymes activity, resulting in higher extracellular polymeric substances (EPS) and coenzyme F₄₂₀ concentrations. X-ray Diffraction (XRD) analysis indicated that chemical of graphene changed insignificantly during the experiment. Meanwhile, with graphene assisted, cells were attached together to form microbial aggregates to facilitate sludge granulation process. Furthermore, the enriched Geobacter and Pseudomonas might perform direct interspecies electron transfer (DIET) with Methanosaeta via biological electrical connection, enhancing the anaerobic degradation of CGW.

The toxicity of graphene and its impacting on bioleaching of metal ions from sewages sludge by Acidithiobacillus sp

The increasing production of graphene raised concerns about their releasing into sewage sludge, however, there is little information about graphene impacting on the growth of bacteria and hence their bioleaching of metal ions from sewages sludge. In this study, we reported that Acidithiobacillus sp., isolated from sewages, were used to bioleach Cu²⁺ and Zn²⁺ from sewages sludge in the presence of graphene. The negative effect on the growth of Acidithiobacillus sp. and dose-dependent were observed in presence of graphene, where the optical density (OD₄₂₀) of the culture decreased from 0.163 to 0.045, while the bioleaching efficiency of Cu²⁺ (70%-16%) and Zn²⁺ (80%-48%) were also reduced when the graphene dose decreased from 50 mg L^-1 to 1 mg L^-1. Furthermore, scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirmed that the direct contacts between graphene and cell at 1 mg L^-1 graphene caused cell membrane disruption, while Acidithiobacillus sp. grew better by forming dense biofilms around the suspended graphene at a 50 mg L^-1. LIVE/DEAD staining further demonstrated that almost no live cells were detected at 1 mg L^-1 graphene. The toxicity of graphene could generally be explained by depending on the concentration of graphene. The new findings provide an insight into dose dependence, which impacted on the growth of Acidithiobacillus sp. and their bioleaching of metal ion from sludge.

Bacterial community and molecular ecological network in response to Cr2O3 nanoparticles in activated sludge system

The potential environmental risks of chromium oxide nanoparticles (Cr₂O₃ NPs) have caused great concerns. However, their possible impacts on activated sludge process are very limited. In this study, we carried out long-term exposure experiments to evaluate the influence of Cr₂O₃ NPs on wastewater nutrient removal, bacterial community and molecular ecological network (MEN) in the sequencing batch reactor (SBR). It was found that 1 mg/L Cr₂O₃ NPs increased the effluent concentrations of NO₃^--N and TP from 6.5 to 15.4 mg/L, and 0.6-2.7 mg/L, indicating the serious deterioration of denitrification and phosphorus removal. Cr₂O₃ NPs significantly decreased the bacterial richness in terms of the number of different OTUs (626 OTUs in Cr₂O₃ samples and 728 OTUs in controls). Detrended correspondence analysis (DCA) showed that the overall taxonomic structure of bacterial community was altered at Cr₂O₃ NPs in activated sludge systems. Further analysis revealed that three genera related to denitrification (Desulfovibrio, Pseudomonas and Hyphomicrobium) and two genera related to phosphorus removal (Accumulibacter and Rhodobacter) decreased significantly, which was consistent with the observed influences of Cr₂O₃ NPs on denitrification and phosphorus removal. MEN analysis showed that the overall architecture of the network under Cr₂O₃ NPs was substantially alerted. β-Proteobacteria, playing an important role in nutrients removal, had less complex interactions in the presence of Cr₂O₃ NPs, which may be associated with the deterioration of denitrification and phosphorus removal. This study provides insights into our understanding of shifts in the bacteria community and their MEN under Cr₂O₃ NPs in activated sludge systems.

Acceleration of organic removal and electricity generation from dewatered oily sludge in a bioelectrochemical system by rhamnolipid addition

Conversion of biomass energy of dewatered oily sludge to electricity is the rate-limiting process in bioelectrochemical system (BES). In this study, 2mgg^-1 rhamnolipids were added to dewatered oily sludge, resulting in a significant enhancement in maximum power density from 3.84±0.37 to 8.63±0.81Wm^-3, together with an increase in total organic carbon (TOC) and total petroleum hydrocarbon (TPH) removal from 24.52±4.30 to 36.15±2.79mgg^-1 and 29.51±3.30 to 39.80±2.47mgg^-1, respectively. Rhamnolipids can also enhance the solubilization and promote the hydrolysis of dewatered oily sludge with increases in SOCD from 14.93±2.44 to 18.40±0.08mgg^-1 and VFAs from 1.02±0.07 to 1.39±0.12mgg^-1. Furthermore, bacteria related to substrate degradation were predominant in dewatered oily sludge, and bacteria related to the sulfate/sulfide cycle were significantly enriched by rhamnolipid addition.

Boosting biomethane yield and production rate with graphene: The potential of direct interspecies electron transfer in anaerobic digestion

Interspecies electron transfer between bacteria and archaea plays a vital role in enhancing energy efficiency of anaerobic digestion (AD). Conductive carbon materials (i.e. graphene nanomaterial and activated charcoal) were assessed to enhance AD of ethanol (a key intermediate product after acidogenesis of algae). The addition of graphene (1.0g/L) resulted in the highest biomethane yield (695.0±9.1mL/g) and production rate (95.7±7.6mL/g/d), corresponding to an enhancement of 25.0% in biomethane yield and 19.5% in production rate. The ethanol degradation constant was accordingly improved by 29.1% in the presence of graphene. Microbial analyses revealed that electrogenic bacteria of Geobacter and Pseudomonas along with archaea Methanobacterium and Methanospirillum might participate in direct interspecies electron transfer (DIET). Theoretical calculations provided evidence that graphene-based DIET can sustained a much higher electron transfer flux than conventional hydrogen transfer.

Performance and microbial community dynamics of electricity-assisted sequencing batch reactor (SBR) for treatment of saline petrochemical wastewater

High-salinity wastewater is often difficult to treat by common biological technologies due to salinity stress on the bacterial community. Electricity-assisted anaerobic technologies have significantly enhanced the treatment performance by alleviating the impact of salinity stress on the bacterial community, but electricity-assisted aerobic technologies have less been reported. Herein, a novel bio-electrochemistry system has been designed and operated in which a pair of stainless iron mesh-graphite plate electrodes were installed into a sequencing batch reactor (SBR, designated as S1) to strengthen the performance of saline petrochemical wastewater under aerobic conditions. The removal efficiency of phenol and chemical oxygen demand (COD) in S1 were 94.1 and 91.2%, respectively, on day 45, which was clearly higher than the removal efficiency of a single SBR (S2) and an electrochemical reactor (S3), indicating that a coupling effect existed between the electrochemical process and biodegradation. A certain amount of salinity (≤8000 mg/L) could enhance the treatment performance in S1 but weaken that in S2. Illumina sequencing revealed that microbial communities in S1 on days 45 and 91 were richer and more diverse than in S2, which suggests that electrical stimulation could enhance the diversity and richness of the microbial community, and reduce the negative effect of salinity on the microorganisms and enrich some salt-adapted microorganisms, thus improve the ability of S1 to respond to salinity stress. This novel bio-electrochemistry system was shown to be an alternative technology for the high saline petrochemical wastewater.

Effect of granular activated carbon on the aerobic granulation of sludge and its mechanism

The granulation of activated sludge and effect of granular activated carbon (GAC) was investigated under the alternative anaerobic and aerobic conditions. The results showed that GAC accelerated the granulation, but had no obvious effect on the bacterial community structure of granules. The whole granulation process could be categorized into three phases, i.e. lag, granulation and granule maturation phase. During lag period GAC provided nuclei for sludge to attach, and thus enhanced the morphological regularization of sludge. During granulation period the granule size increased significantly due to the growth of bacteria in granules. GAC reduced the compression caused by the inter-particle collisions and thus accelerate the granulation. GAC has no negative effect on the performance of SBR, and thus efficient simultaneous removal of COD, nitrogen and phosphorus were obtained during most of the operating time.

Transformation of chlorpyrifos in integrated recirculating constructed wetlands (IRCWs) as revealed by compound-specific stable isotope (CSIA) and microbial community structure analysis

Carbon isotope analysis and 454 pyrosequencing methods were used to investigate in situ biodegradation of chlorpyrifos during its transport through three model integrated recirculating constructed wetlands (IRCWs). Results show that plant and Fe-impregnated biochar promoted degradation of chlorpyrifos and its metabolite 3,5,6-trichloro-2-pyridinol (TCP). Carbon isotope ratios in the IRCWs shifted to -31.24±0.58‰ (IRCW1, plant free), -26.82±0.60‰ (IRCW2, with plant) and -24.76±0.94‰ (IRCW3, with plant and Fe-biochar). The enrichment factors (Ɛ_bulk,c) were determined as -0.69±0.06‰ (IRCW1), -0.91±0.07‰ (IRCW2) and -1.03±0.09‰ (IRCW3). Microbial community analysis showed that IRCW3 was dominated by members of Bacillus, which can utilize and degrade chlorpyrifos. These results reveal that plant and Fe-biochar can induce carbon isotope fractionation and have a positive impact on the chlorpyrifos degradation efficiency by influencing the development of beneficial microbial communities.

Responses of Bacterial Communities to CuO Nanoparticles in Activated Sludge System

The main objectives of this study were to investigate the influence of copper oxide nanoparticles (CuO NPs) on wastewater nutrient removal, bacterial community and molecular ecological network in activated sludge. The results showed that long-term exposure to 1 mg/L CuO NPs induced an increase of effluent concentrations of ammonia and total phosphorus, which was consistent with the inhibition of enzyme activities of ammonia monooxygenase, nitrite oxidoreductase, exopolyphosphatase, and polyphosphate in the presence of CuO NPs. MiSeq sequencing data indicated that CuO NPs significantly decreased the bacterial diversity and altered the overall bacterial community structure in activated sludge. Some genera involved in nitrogen and phosphorus removal, such as Nitrosomonas, Acinetobacter, and Pseudomonas decreased significantly. Molecular ecological network analysis showed that network interactions among different phylogenetic populations were markedly changed by CuO NPs. For example, β-Proteobacteria, playing an important role in nutrients removal, had less complex interactions in the presence of CuO NPs. These shifts of the abundance of related genera, together with the network interactions may be associated with the deterioration of ammonia and phosphorus removal. This study provides insights into our understanding of shifts in the bacteria community and their molecular ecological network under CuO NPs in activated sludge systems.

Environmental dynamics of metal oxide nanoparticles in heterogeneous systems: A review

Metal oxide nanoparticles (MNPs) have been used for many purposes including water treatment, health, cosmetics, electronics, food packaging, and even food products. As their applications continue to expand, concerns have been mounting about the environmental fate and potential health risks of the nanoparticles in the environment. Based on the latest information, this review provides an overview of the factors that affect the fate, transformation and toxicity of MNPs. Emphasis is placed on the effects of various aquatic contaminants under various environmental conditions on the transformation of metal oxides and their transport kinetics - both in homogeneous and heterogeneous systems - and the effects of contaminants on the toxicity of MNPs. The presence of existing contaminants decreases bioavailability through hetero-aggregation, sorption, and/or complexation upon an interaction with MNPs. Contaminants also influence the fate and transport of MNPs and exhibit their synergistic toxic effects that contribute to the extent of the toxicity. This review will help regulators, engineers, and scientists in this field to understand the latest development on MNPs, their interactions with aquatic contaminants as well as the environmental dynamics of their fate and transformation. The knowledge gap and future research needs are also identified, and the challenges in assessing the environmental fate and transport of nanoparticles in heterogeneous systems are discussed.

Recent advances in nanomaterials for water protection and monitoring

Nanomaterials (NMs) for adsorption, catalysis, separation, and disinfection are scrutinized. NMs-based sensor technologies and environmental transformations of NMs are highlighted.

Nano-graphene induced positive effects on methanogenesis in anaerobic digestion

The effects of nano-graphene on methanogenesis in anaerobic digestion was investigated. Short-term results showed that graphene (30 and 120mg/L) had significantly positive effects on methane production rate, which increased by 17.0% and 51.4%. Further investigation indicated that acetate-consuming methanogenesis was enhanced. The failure of quinones to replicate graphene stimulation effects on methanogenesis suggested that graphene did not function as electron shuttles. After 55 day's operation at room temperature (from 20 to 10°C, the methane production rate with 30mg/L graphene was 14.3% higher than that of the control, while 120mg/L graphene showed a slight inhibition on methane yield. Illumina sequencing data showed that the archaeal community structure remained fairly constant as the incubated sludge with graphene at low temperature, in which Methanoregula, Methanosaeta and Methanospirillum were the dominant species. Besides, Geobacter enrichment was observed with graphene, suggesting that the direct interspecies electron transfer might be promoted.