Understanding the coordination behavior of antibiotics: Take tetracycline as an example.

Gaining insight into the occurrence states of residual antibiotics is crucial to demystify their environmental behavior. However, the complexation of heteroatoms functioned on antibiotic molecules to metal ions in the water environment is not fully understood. This study reports that a fluorescence response was unexpectedly triggered by tetracycline (TC) and Al3+, serving as solid evidence to visualize the Al3+-TC coordination reaction. Differential electron absorption spectroscopy shows a quantifiable signal of the redshifted n-π* transition with a coordination reaction, which is also proportional to the fluorescence. The occurrence of Al3+-complexed TC also caused a split in retention time in liquid chromatogram. The TC ligands were re-released in the presence of stronger ligands competing for central Al3+. The complex ratio of Al3+-TC is confirmed to be 1:1 using Job's plot with a stability constant of 1.01 × 106. Quantum chemical computations coupled with Gibbs free energy analysis simulated the formation of octahedral Al3+-TC configuration through a spontaneous bidentate chelation. This study helps convey a broad consensus and opens a new door in the mechanistic study of metal-involved antibiotic transformation process, leading to a better understanding that can ultimately be essential to reach the final goal of alleviating the antibiotic crisis.

Polydopamine and calcium functionalized fiber carrier for enhancing microbial attachment and Cr(VI) resistance.

The formation of biofilm determines the performance and stability of biofilm system. Increasing the hydrophilicity of the carrier surface could efficiently accelerate the attachment and growth of microorganisms. Here, the surface of polypropylene (PP) fiber carrier was modified with polydopamine (PDA) and calcium (Ca(II)) to enhance microbial attachment and toxicity resistance. The results of surface characteristic confirmed the self-polymerization of PDA and the chelation mechanism of Ca(II). Subsequently, the biofilm formation experiments were conducted in sequencing batch biofilm reactors using both normal and chromium-containing wastewater. The biofilm on the surface of the modified carrier exhibited better nitrogen removal and Cr(VI) reduction ability. The biomass of the modified carrier was significantly increased, and the maximum microbial attachment amounts in normal wastewater and chrome-containing wastewater were 1153.34 and 511.78 mg/g carrier, respectively. Furthermore, the confocal laser scanning microscope (CLSM) indicated that the modified carrier coated with PDA and Ca(II) were both biocompatible, and the cell activity was significantly increased. 16S rRNA sequencing results showed that the modified carrier efficiently enriched both denitrification bacteria (Thauera and Flavobacterium) and chrome-reducing bacteria (Simplicispira and Arenimonas) to improve system stability and Cr(VI) resistance. Microbial phenotype prediction based on BugBase analysis further verified the enrichment effect of modified carriers on microorganisms responsible for biofilm formation and oxidative stress resistance. Overall, this work proposed a novel functional carrier that could provide references for advancing the application of biofilm systems in wastewater treatment.

Managing microbial sulfur disproportionation for optimal sulfur autotrophic denitrification in a pilot-scale elemental sulfur packed-bed bioreactor.

Elemental sulfur packed-bed (S0PB) bioreactors for autotrophic denitrification have gained more attention in wastewater treatment due to their organic carbon-free operation, low operating cost, and minimal carbon emissions. However, the rapid development of microbial S0-disproportionation (MS0D) in S0PB reactor during deep denitrification poses a significant drawback to this new technology. MS0D, the process in which sulfur is used as both an electron donor and acceptor by bacteria, plays a crucial role in the microbial-driven sulfur cycle but remains poorly understood in wastewater treatment setups. In this study, we induced MS0D in a pilot-scale S0PB reactor capable of denitrifying over 1000 m3/d nitrate-containing wastewater. Initially, the S0PB reactor stably removed 6.6 mg-NO3--N/L nitrate at an empty bed contact time (EBCT) of 20 mins, which was designated the S0-denitrification stage. To induce MS0D, we reduced the influent nitrate concentrations to allow deep nitrate removal, resulted in the production of large quantities of sulfate and sulfide (SO42-:S2- 3.2 w/w). Meanwhile, other sulfur-heterologous electron acceptors (SHEAs), e.g., nitrite and DO, were also kept at trace levels. The negative correlations between the SHEAs concentrations and the sulfide productions indicated that the absence of SHEAs was a primary inducing factor to MS0D. The microbial community drastically diverged in response to the depletion of SHEAs during the switch from S0-denitrification to S0-disproportionation. An evident enrichment of sulfur-disproportionating bacteria (SDBs) was found at the S0-disproportionation stage, accompanied by the decline of sulfur-oxidizing bacteria (SOBs). In the end, we discovered that shortening the EBCT and increasing the reflux ratio could inhibit sulfide production by reducing it from 43.9 mg/L to 3.2 mg/L or 25.5 mg/L. In conclusion, our study highlights the importance of considering MS0D when designing and optimizing S0PB reactors for sustainable autotrophic sulfur denitrification in real-life applications.

Carbon source recovery from waste sludge reduces greenhouse gas emissions in a pilot-scale industrial wastewater treatment plant.

Carbon cycle regulation and greenhouse gas (GHG) emission abatement within wastewater treatment plants (WWTPs) can theoretically improve sustainability. Currently, however, large amounts of external carbon sources used for deep nitrogen removal and waste sludge disposal aggravate the carbon footprint of most WWTPs. In this pilot-scale study, considerable carbon was preliminarily recovered from primary sludge (PS) through short-term (five days) acidogenic fermentation and subsequently utilized on-site for denitrification in a wool processing industrial WWTP. The recovered sludge-derived carbon sources were excellent electron donors that could be used as additional carbon supplements for commercial glucose to enhance denitrification. Additionally, improvements in carbon and nitrogen flow further contributed to GHG emission abatement. Overall, a 9.1% reduction in sludge volatile solids was achieved from carbon recovery, which offset 57.4% of external carbon sources, and the indirect GHG emissions of the target industrial WWTP were reduced by 8.05%. This study demonstrates that optimizing the allocation of carbon mass flow within a WWTP has numerous benefits.

A tartrate-EDTA-Fe complex mediates electron transfer and enhances ammonia recovery in a bioelectrochemical-stripping system.

Traditional bioelectrochemical systems (BESs) coupled with stripping units for ammonia recovery suffer from an insufficient supply of electron acceptors due to the low solubility of oxygen. In this study, we proposed a novel strategy to efficiently transport the oxidizing equivalent provided at the stripping unit to the cathode by introducing a highly soluble electron mediator (EM) into the catholyte. To validate this strategy, we developed a new kind of iron complex system (tartrate-EDTA-Fe) as the EM. EDTA-Fe contributed to the redox property with a midpoint potential of -0.075 V (vs. standard hydrogen electrode, SHE) at pH 10, whereas tartrate acted as a stabilizer to avoid iron precipitation under alkaline conditions. At a ratio of the catholyte recirculation rate to the anolyte flow rate (RC-A) of 12, the NH4 +-N recovery rate in the system with 50 mM tartrate-EDTA-Fe complex reached 6.9 ±â€¯0.2 g N m-2 d-1, approximately 3.8 times higher than that in the non-EM control. With the help of the complex, our system showed an NH4 +-N recovery performance comparable to that previously reported but with an extremely low RC-A (0.5 vs. 288). The strategy proposed here may guide the future of ammonia recovery BES scale-up because the introduction of an EM allows aeration to be performed only at the stripping unit instead of at every cathode, which is beneficial for the system design due to its simplicity and reliability.

Reinforcement of denitrification in a biofilm electrode reactor with immobilized polypyrrole/anthraquinone-2,6-disulfonate composite cathode.

In biofilm electrode reactors (BER), good nitrate removal performance can be achieved through cooperation of heterotrophic and hydrogen autotrophic denitrification under low carbon/nitrogen conditions. In this study, we proposed a more multifunctional composite cathode, which combine immobilized anthraquinone-2,6-disulphonic disodium salt (AQDS) with polypyrrole (PPy) by electrochemical polymerization-doping method. The nitrate removal performance in BER with PPy/AQDS composite cathode was obviously improved, the nitrate removal rate (4.96 mg/L·h) was almost 2.0 times higher than the control BER system, and relatively stabled nitrate removal efficiency (≥90.0%) was also achieved even as the COD/N of 2.50. Compared with the bare graphite felt, PPy/AQDS coating cathode showed much better electrocatalytic activities, which was more advantageous for in situ production of H2 to support hydrogen autotrophic denitrification process. The PPy-bound AQDS could also act as electron intermediaries, which is beneficial to greatly promote indirect electron process between the denitrifiers and nitrate. Moreover, the PPy/AQDS composite layer formed many particles for improving the specific surface area and bio-attachment site for bacterial attachment, which was conducive for the proliferation of microorganisms and denitrification efficiency. The ratio of biofilm and electrode of PPy/AQDS biocathode was 0.32 ± 0.08, which was 2.46 times than bare electrode (0.13 ± 0.06). Furthermore, enrichment of specific denitrifiers and enhancement of denitrifying enzyme activity was obtained using PPy/AQDS treated electrode, the much higher relative abundance of Thauera of PPy/AQDS biocathode was 1.58 times to the application of bare graphite felt.

Fabrication of Pd/Sludge-biochar electrode with high electrochemical activity on reductive degradation of 4-chlorophenol in wastewater.

Effective treatment and utilization of sludge contribute to achieve conventional carbon emission reduction and resource recovery, which is of great significance to realize carbon neutralization of WWTPs. Sludge carbonization derived biochar has attracted more interest because of high potential as catalytic materials. Therein, sludge-derived electrode exhibits a promising potential in the case of sludge utilization for electrocatalysis, however, electrocatalytic performance of the already reported sludge-derived electrode is unsatisfactory due to insufficient active sites. In this study, an efficient Pd/sludge-biochar loaded foam nickel (Pd-SAC@Ni) was successfully fabricated using simple pyrolysis and solidification method, and exhibited remarkable electrocatalytic performance for 4-chlorophenol (4-CP) degradation. Furthermore, the morphology, element distribution and crystal composition were characterized by SEM, EDS, XPS and XRD. The Pd-SAC@Ni electrode exhibited superior electrocatalytic performance than Ni, SAC@Ni, Pd-Ni electrodes. The reduction rate of 98.9% was achieved at current density of 5 mA cm-2, 4-CP concentration of 0.8 mM and initial pH of 7.0. Also, Pd-SAC@Ni electrode showed desirable reusability and achieved 98% of 4-CP removal after multiple runs of experiments. Moreover, the active hydrogen species (H*) generation capacity of electrodes was determined using tert-butanol (TBA) as trapping agent. The mechanism analysis demonstrated that direct reduction process and indirect reduction process both involved in the 4-CP degradation process, and their contribution were 19.5% and 80.5%, respectively. Then, the intermediates formed in the electrochemical degradation of 4-CP were revealed by HPLC and the plausible degradation pathway was proposed. This study provides a cost-effective approach for preparing sludge biochar electrode, and explored a novel way to promote resourceful utilization of sludge for carbon neutrality.

Imine Reduction with Me2S-BH3.

Although there exists a variety of different catalysts for hydroboration of organic substrates such as aldehydes, ketones, imines, nitriles etc., recent evidence suggests that tetra-coordinate borohydride species, formed by activation, redistribution, or decomposition of boron reagents, are the true hydride donors. We then proposed that Me2S-BH3 could also act as a hydride donor for the reduction of various imines, as similar compounds have been observed to reduce carbonyl substrates. This boron reagent was shown to be an effective and chemoselective hydroboration reagent for a wide variety of imines.

Application of different redox mediators induced bio-promoters to accelerate the recovery of denitrification and denitrifying functional microorganisms inhibited by transient Cr(VI) shock.

The transient hexavalent chromium (Cr(VI)) shock may directly inhibit the denitrification process of municipal wastewater treatment plants (WWTPs), which is difficult to recover in a short time. This study developed four nontoxic bio-promoters (combination of L-cysteine, flavin adenine dinucleotide (FAD), biotin, cytokinin and different redox mediators) to quickly restore the denitrification performance after high-loading Cr(VI) suppressing. After feeding with 100 mg/L of Cr(VI) for 42 cycles (T, 4 h), the removal efficiency of nitrate was reduced by 85.00%, and nitrite was accumulated simultaneously. The denitrification performance was recovered quickly with the addition of bio-promoters, introducing redox mediators showed noticeable superiority on the bio-inhibition release. Compared with sodium humate and riboflavin, the AQDS induced bio-promoter achieved the best nitrate removal recovery performance within only 28 T, and the recovery rate was 2.16 times faster than the natural recovery. Microbial analysis showed that Cr(VI) specially inhibited napA-type denitrifiers, and the OTU numbers sharply dropped by 48.74%. Redox mediators induced bio-promoters could effectively recover the abundance of napA-type and nirS-type denitrifying microorganisms, which was consistent with the change of nitrate removal efficiency. This study offers a cost-effective approach to deal with Cr(VI) shock problem, which may promote the development of bio-promoters for WWTPs.

Optimal control towards sustainable wastewater treatment plants based on multi-agent reinforcement learning.

Wastewater treatment plants (WWTPs) are designed to eliminate pollutants and alleviate environmental pollution resulting from human activities. However, the construction and operation of WWTPs consume resources, emit greenhouse gases (GHGs) and produce residual sludge, thus require further optimization. WWTPs are complex to control and optimize because of high non-linearity and variation. This study used a novel technique, multi-agent deep reinforcement learning (MADRL), to simultaneously optimize dissolved oxygen (DO) and chemical dosage in a WWTP. The reward function was specially designed from life cycle perspective to achieve sustainable optimization. Five scenarios were considered: baseline, three different effluent quality and cost-oriented scenarios. The result shows that optimization based on LCA has lower environmental impacts compared to baseline scenario, as cost, energy consumption and greenhouse gas emissions reduce to 0.890 CNY/m3-ww, 0.530 kWh/m3-ww, 2.491 kg CO2-eq/m3-ww respectively. The cost-oriented control strategy exhibits comparable overall performance to the LCA-driven strategy since it sacrifices environmental benefits but has lower cost as 0.873 CNY/m3-ww. It is worth mentioning that the retrofitting of WWTPs based on resources should be implemented with the consideration of impact transfer. Specifically, LCA-SW scenario decreases 10 kg PO4-eq in eutrophication potential compared to the baseline within 10 days, while significantly increases other indicators. The major contributors of each indicator are identified for future study and improvement. Last, the authors discussed that novel dynamic control strategies required advanced sensors or a large amount of data, so the selection of control strategies should also consider economic and ecological conditions. In a nutshell, there are still limitations of this work and future studies are required.

Evaluating the effect of fenton pretreated pyridine wastewater under different biological conditions: Microbial diversity and biotransformation pathways.

Pyridine contamination poses a significant threat to human and environmental health. Due to the presence of nitrogen atom in the pyridine ring, the pi bond electrons are attracted toward it and make difficult for pyridine treatment with biological and chemical methods. In this study, coupling Fenton treatment with different biological process was designed to enhance pyridine biotransformation and further mineralization. After Fenton oxidation process optimized, pretreated pyridine was evaluated under three biological (anaerobic, aerobic and microaerobic) operating conditions. Under optimum Fenton oxidation, pyridine (30-75%) and TOC (5-25%) removal efficiencies were poor. Biological process alone also showed insignificant removal efficiency, particularly anaerobic (pyridine = 8.2%; TOC = 5.3%) culturing condition. However, combining Fenton pretreatment with biological process increased pyridine (93-99%) and TOC (87-93%) removals, suggesting that hydroxyl radical generated during Fenton oxidation enhanced pyridine hydroxylation and further mineralization in the biological (aerobic > microaerobic > anaerobic) process. Intermediates were analyzed with UPLC-MS and showed presence of maleic acid, pyruvic acid, glutaric dialdehyde, succinic semialdehyde and 4-formylamino-butyric acid. High-throughput sequencing analysis also indicated that Proteobacteria (35-43%) followed by Chloroflexi (10.6-24.3%) and Acidobacteria (8.0-29%) were the dominant phyla detected in the three biological treatment conditions. Co-existence of dominant genera under aerobic/microaerobic (Nitrospira > Dokdonella > Caldilinea) and anaerobic (Nitrospira > Caldilinea > Longilinea) systems most probably play significant role in biotransformation of pyridine and its intermediate products. Overall, integrating Fenton pretreatment with different biological process is a promising technology for pyridine treatment, especially the combined system enhanced anaerobic (>10 times) microbial pyridine biotransformation activity.

Challenges and opportunities for the biodegradation of chlorophenols: Aerobic, anaerobic and bioelectrochemical processes.

Chlorophenols (CPs) are highly toxic and refractory contaminants which widely exist in various environments and cause serious harm to human and environment health and safety. This review provides comprehensive information on typical CPs biodegradation technologies, the most green and benign ones for CPs removal. The known aerobic and anaerobic degradative bacteria, functional enzymes, and metabolic pathways of CPs as well as several improving methods and critical parameters affecting the overall degradation efficiency are systematically summarized and clarified. The challenges for CPs mineralization are also discussed, mainly including the dechlorination of polychlorophenols (poly-CPs) under aerobic condition and the ring-cleavage of monochlorophenols (MCPs) under anaerobic condition. The coupling of functional materials and degraders as well as the operation of sequential anaerobic-aerobic bioreactors and bioelectrochemical system (BES) are promising strategies to overcome some current limitations. Future perspective and research gaps in this field are also proposed, including the further understanding of microbial information and the specific role of materials in CPs biodegradation, the potential application of innovative biotechnologies and new operating modes to optimize and maximize the function of the system, and the scale-up of bioreactors towards the efficient biodegradation of CPs.

Rapid recovery of inhibited denitrification with cascade Cr(VI) exposure by bio-accelerant: Characterization of chromium distributions, EPS compositions and denitrifying communities.

Hexavalent chromium (Cr(VI)) may inhibit denitrification in biological wastewater treatment systems, and the inhibited denitrification process is difficult to recover in a short time. This study explored Cr(VI) cascade impact (20-125 mg L-1) on denitrification and developed one nontoxic biological accelerant (combination of L-cysteine, flavin adenine dinucleotide, biotin and cytokinin) for denitrification recovery. The results showed that NO3--N removal efficiency decreased from 75.7% to 21.5% when Cr(VI) concentration increased from 80 to 125 mg L-1. Addition of accelerant could effectively promote the removal of NO3--N, and observably reduce the recovery time (42 T) compared with natural recovery (63 T). Furthermore, the main site of Cr(VI) reduction and Cr(III) immobilization was located in the intercellular compartment of the biofilm. Microbes produced more tightly bound extracellular polymeric substances (TB-EPS) to protect them from toxicity under the low Cr(VI) concentrations, while low EPS was secreted when Cr(VI) concentration was higher than 60 mg L-1. Compared to natural recovery system, bio-accelerant addition was beneficial to the recovery of denitrifiers activities, especially for the bacteria containing nirS gene. The results facilitated an understanding of Cr(VI) impact on denitrification, and the proposed bio-accelerant can be potentially applied to heavy metal shock-loading emergency situations.

Biotransformation of 4-Hydroxybenzoic Acid under Nitrate-Reducing Conditions in a MEC Bioanode.

4-Hydroxybenzoic acid (HBA) is commonly found at high concentrations in waste streams generated by the thermochemical conversion of lignocellulosic biomass to bio-oils and biofuels. The objective of this study was to systematically assess the biotransformation of HBA in the bioanode of a microbial electrolysis cell (MEC) for the production of renewable cathodic H2. A mixed, denitrifying culture, enriched with HBA as the sole electron donor, was used as the anode inoculum. MEC electrochemical performance, H2 yield, HBA biotransformation pathways and products, and the bioanode suspended and biofilm microbial communities were examined. In the absence of nitrate, 60%-100% HBA was converted to phenol, which persisted, resulting in very limited exoelectrogenesis. Under nitrate-reducing conditions, complete HBA degradation was achieved in the MEC bioanode with very low phenol production, resulting in the production of cathodic H2. The predominant bacterial genus in the MEC bioanode (relative abundance 33.4%-41.9%) was the denitrifier Magnetospirillum, which uses the benzoyl-CoA pathway to degrade aromatic compounds. Geobacter accounted for 5.9-7.8% of the MEC bioanode community. Thus, active nitrate reduction in the MEC bioanode led to complete HBA degradation, resulting in a higher extent of exoelectrogenesis and cathodic H2 production. The results of this study provide mechanistic insights into a productive use of HBA and other phenolic compounds typically found in waste streams resulting from the thermochemical conversion of lignocellulosic biomass to biofuels.

Wire-drawing process with graphite lubricant as an industrializable approach to prepare graphite coated stainless-steel anode for bioelectrochemical systems.

Carbon coated stainless-steel (SS) electrode has been suggested to be a powerful composite electrode with high conductivity, excellent biocompatibility and good mechanical strength, which is promising for scaling up the bioelectrochemical systems (BESs). However, the already reported carbon coating methods were independent on the production of SS material. Additional steps and investment of equipment for carbon coating are costly, and the industrialization of these carbon coating processes remains challenging. In this study, we report an industrializable carbon coating approach that was embedded into the production line of the SS wire, which was realized through a wire-drawing process with graphite emulsion as the lubricant and carbon source. We found the slide of SS wire through the dies was essential for the graphite coating in terms of loading amount and stability. When the graphite coated SS wire was prepared as the anode and operated in a BESs, the current density reached 1.761 ± 0.231 mA cm-2, which was 20 times higher than that without graphite coating. Biomass analysis was then conducted, confirming the superior bioelectrochemical performance was attributed to the improvement of biocompatibility by the graphite coating layer. Furthermore, graphite coating by the wire-drawing process was systematically compared with the existing methods, which showed a comparable or even better bioelectrochemical performance but with extremely low cost (0.036 $·m-2) and seconds level of the time consumption. Overall, this study offers a cost-effective and industrializable approach to preparing graphite coated SS electrode, which may open up great opportunities to promote the development of BESs at large scale.

Bioelectrochemical degradation of monoaromatic compounds: Current advances and challenges.

Monoaromatic compounds (MACs) are typical refractory organic pollutants which are existing widely in various environments. Biodegradation strategies are benign while the key issue is the sustainable supply of electron acceptors/donors. Bioelectrochemical system (BES) shows great potential in this field for providing continuous electrons for MACs degradation. Phenol and BTEX (Benzene, Toluene, Ethylbenzene and Xylenes) can utilize anode to enhance oxidative degradation, while chlorophenols, nitrobenzene and antibiotic chloramphenicol (CAP) can be efficiently reduced to less-toxic products by the cathode. However, there still have several aspects need to be improved including the scale, electricity output and MACs degradation efficiency of BES. This review provides a comprehensive summary on the BES degradation of MACs, and discusses the advantages, future challenges and perspectives for BES development. Instead of traditional expensive dual-chamber configurations for MACs degradation, new single-chamber membrane-less reactors are cost-effective and the hydrogen generated from cathodes may promote the anode degradation. Electrode materials are the key to improve BES performance, approaches to increase the biofilm enrichment and conductivity of materials have been discussed, including surface modification as well as composition of carbon and metal-based materials. Besides, the development and introduction of functional microbes and redox mediators, participation of sulfur/hydrogen cycling may further enhance the BES versatility. Some critical parameters, such as the applied voltage and conductivity, can also affect the BES performance, which shouldn't be overlooked. Moreover, sequential cathode-anode cascaded mode is a promising strategy for MACs complete mineralization.

A breathable, biodegradable, antibacterial, and self-powered electronic skin based on all-nanofiber triboelectric nanogenerators.

Mimicking the comprehensive functions of human sensing via electronic skins (e-skins) is highly interesting for the development of human-machine interactions and artificial intelligences. Some e-skins with high sensitivity and stability were developed; however, little attention is paid to their comfortability, environmental friendliness, and antibacterial activity. Here, we report a breathable, biodegradable, and antibacterial e-skin based on all-nanofiber triboelectric nanogenerators, which is fabricated by sandwiching silver nanowire (Ag NW) between polylactic-co-glycolic acid (PLGA) and polyvinyl alcohol (PVA). With micro-to-nano hierarchical porous structure, the e-skin has high specific surface area for contact electrification and numerous capillary channels for thermal-moisture transfer. Through adjusting the concentration of Ag NW and the selection of PVA and PLGA, the antibacterial and biodegradable capability of e-skins can be tuned, respectively. Our e-skin can achieve real-time and self-powered monitoring of whole-body physiological signal and joint movement. This work provides a previously unexplored strategy for multifunctional e-skins with excellent practicability.

Shift of bacterial community and denitrification functional genes in biofilm electrode reactor in response to high salinity.

High salinity suppresses denitrification by inhibiting microorganism activities. The shift of microbial community and denitrification functional genes under salinity gradient was systematically investigated in a biofilm electrode reactor (BER) and biofilm reactor (BR) systems. Denitrification efficiency of both BER and BR was not significantly inhibited during the period of low salinity (0-2.0%). As the salinity increased to 2.5%, BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance, and the effluent NO3--N was lower than 1.5 mg/L. High salinity (>2.5%) impoverished microbial diversity and altered the microbial community in both BER and BR. However, two genera Methylophaga and Methyloexplanations were enriched in BER due to electrochemical stimulation, which can tolerate high salinity (>3.0%). The relative abundance of Methylophaga in BER was almost 10 times as much as in BR. Paracoccus is a hydrogen autotrophic denitrifier, which was obviously inhibited with 1.0% NaCl. The hetertrophic denitrifiers were primarily responsible for the nitrate removal in the BER compared to the autotrophic denitrifiers. The abundance and proportion of denitrifying functional genes confirmed that main denitrifiers shift to salt-tolerant species (nirK-type denitrifiers) to reduce the toxic effects. The napA (2.2 × 108 to 6.5 × 108 copies/g biofilm) and nosZ (2.2 × 107 to 4.4 × 107 copies/g biofilm) genes were more abundant in BER compared to BR's, which was attributed to the enrichment of Methylophaga alcalica and Methyloversatilis universalis FAM5 in the BER. The results proved that BER had greater denitrification potential under high salinity (>2.0%) stress at the molecular level.

A dicyclic-type electrode-based biofilm reactor for simultaneous nitrate and Cr(VI) reduction.

A dicyclic-type electrode-based biofilm-electrode reactor (BER) was investigated for simultaneous removal of nitrate and Cr(VI). In the absence of Cr(VI), almost complete denitrification of 50 mg/L NO3--N was achieved at a very low C/N ratio of 0.8 with the optimal current of 50 mA. Cr(VI) was removed by biological reduction and co-precipitation when Cr(VI) was taken as the only electron acceptor, and the removal efficiencies of Cr(VI) were 99.8%. In the coexistent system of nitrate and Cr(VI), nitrate removal was the result of the cooperation of hydrogenotrophic denitrification and heterotrophic denitrification. The methanol and H2 were also used as electron donors for biological reduction Cr(VI). The denitrification process was slightly inhibited by 1.00 mg/L Cr(VI) and 94.15% removal efficiency was achieved at current = 50 mA and HRT = 8 h. The present results show that the biofilm-electrode reactor is an effective way to simultaneous remove co-contaminants.

Effects of salinity and COD/N on denitrification and bacterial community in dicyclic-type electrode based biofilm reactor.

A dicyclic-type electrode based biofilm electrode reactor (BER) was developed for advanced nitrate removal from saline municipal wastewater. The denitrification efficiency was evaluated with a synthetic feed (NO3--N, 20 mg L-1) under different salinity and COD to nitrogen ratios (COD/N). As the salinity increased from 0% to 1.0%, the denitrification performance of both the traditional biofilm reactor (BR) and BER was inhibited; however, the BER showed better adaptation and ability to recover. The BER achieved a high nitrate removal efficiency (≥90%) at a salinity of 1.0% and a low COD/N of 2.5 (theoretical stoichiometric 2.86 ignoring microbial growth). The abundance of Methylotenera mobilis in BR and Clostridium sticklandii in BER was higher than in the initial sludge sample used as inoculum. Likewise, the abundance of napA, nirS and nosZ genes increased as the COD/N further decreased. Under high salinity stress, the BER had a higher denitrification efficiency and the consumption of the organic carbon source (i.e., methanol) was reduced compared to BR. The cooperation between heterotrophic and autotrophic denitrifiers in the BER system provides a more efficient and feasible solution for nitrate removal from saline municipal wastewater.