High recycling efficiency and elemental sulfur purity achieved in a biofilm formed membrane filtration reactor

Effect of magnetic powder (Fe3O4) on heterotrophic-sulfur autotrophic denitrification efficiency and electron transport system activity for marine recirculating aquacultural wastewater treatment

As an efficient nitrogen removal process, heterotrophic-sulfur autotrophic denitrification (HSAD) has attracted extensive attention in wastewater treatment. However, the effects of magnetic powder (Fe3O4) on the electron transport activity in HSAD process remain unclear. Therefore, in this study, a heterotrophic-sulfur autotrophic denitrification system was established to remove nitrogen from marine recirculating aquacultural wastewater for evaluating the effects of Fe3O4. At the optimal Fe3O4 concentration of 50 mg/L, the nitrogen removal efficiency reached 100% with lower sulfate accumulation, and the start-up time was shortened. The assays of denitrifying enzymes and electron transport system activity showed that Fe3O4 improved the activities of nitrate and nitrite reductases, and increased the efficiency of electron transport. Microbial community analysis revealed that Fe3O4 enriched heterotrophic denitrifier Thauera and sulfur autotrophic denitrifier Canditatus Thiobios, and thus enhanced denitrification efficiencies. This study demonstrated that Fe3O4 is an efficient denitrification accelerator in HSAD for treating marine recirculating aquacultural wastewater.

Sulfide addition accelerates anammox sludge granulation and promotes microbial cooperation

The granular anaerobic ammonium oxidation (anammox) system has attractive advantages in tolerance to environmental-stress and enhancement of nitrogen removal capacity. Sulfide addition can improve nitrogen removals in anammox systems via inducing sulfur denitrification, yet its function in the improvement of the property of anammox granular sludge remains unclear. Herein, we investigated the variations in the morphological and microbial properties of the anammox sludge response to different sulfide concentrations (Na2S: 10-100 mg/L) through a long-term experiment. By comparing the sludge diameter and heme c content, it comes that a relatively low sulfide (S/N [nitrate] molar ratio of 0.18-0.50) significantly promoted the average diameter and heme c concentration of sludge by 25-175 % and 75-95 %, respectively, compared to that of both without sulfide addition and a high sulfide addition (S/N > 0.85). This enhancement is primarily because a low amount of sulfide had stimulated the secretion of extracellular polymeric substance, induced slight biogenic sulfur accumulation as microbial nuclei, and facilitated the appropriate amount of filamentous bacteria proliferation. Microbial metabolism functions analyses revealed a robust granular anammox coupled with sulfur denitrification in the sulfide-mediated anammox reactor, and the assembled granules exhibited exceptional tolerance to environmental stress. Significantly, the anammox bacteria (Candidatus_Brocadia) dominating the granules displayed satisfactory anammox activity (21.8 ± 2.1 mg N/g VSS h), and their produced nitrate was efficiently removed by the sulfur-oxidizing bacteria (Thiobacillus) that predominantly occurred in the flocs. This collaboration ensured an efficient sulfide-mediated anammox granules system, achieving nitrogen removal efficiency exceeding 95 %. These results highlight the function of sulfide in improving the morphological property of anammox sludge as well as the creation of a favorable ecological niche for the functional microorganism, which is important to maintain the efficiency and robustness of the anammox process in treating wastewater.

Sulfur autotrophic denitrification as an efficient nitrogen removals method for wastewater treatment towards lower organic requirement: A review

The sulfur autotrophic denitrification (SADN) process is an organic-free denitrification process that utilizes reduced inorganic sulfur compounds (RISCs) as the electron donor for nitrate reduction. It has been proven to be a cost-effective and environment-friendly approach to achieving carbon neutrality in wastewater treatment plants. However, there is no consensus on whether SADN can become a dominant denitrification process to treat domestic wastewater or industrial wastewater if organic carbon is desired to be saved. Through a comprehensive summary of the SADN process and extensive discussion of state-of-the-art SADN-based technologies, this review provides a systematic overview of the potential of the SADN process as a sustainable alternative for the heterotrophic denitrification (HD) process (organic carbons as electron donor). First, we introduce the mechanism of the SADN process that is different from the HD process, including its transformation pathways based on different RISCs as well as functional bacteria and key enzymes. The SADN process has unique theoretical advantages (e.g., economy and carbon-free, less greenhouse gas emissions, and a great potential for coupling with novel autotrophic processes), even if there are still some potential issues (e.g., S intermediates undesired production, and relatively slow growth rate of sulfur-oxidizing bacteria [SOB]) for wastewater treatment. Then we present the current representative SADN-based technologies, and propose the outlooks for future research in regards to SADN process, including implement of coupling of SADN with other nitrogen removal processes (e.g., HD, and sulfate-dependent anaerobic ammonium oxidation), and formation of SOB-enriched biofilm. This review will provide guidance for the future applications of the SADN process to ensure a robust-performance and chemical-saving denitrification for wastewater treatment.

Response of sulfur-metabolizing biofilm to external sulfide in element sulfur-based denitrification packed-bed reactor

Dosing sulfide into the sulfur-packed-bed (S0PB) has great potential to enhance the denitrification efficiency by providing compensatory electron donors, however, the response of sulfur-metabolizing biofilm to various sulfide dosages has never been investigated. In this study, the S0PB reactor was carried out with increasing sulfide dosages by 3.6 kg/m3/d, presenting a decreasing effluent nitrate from 14.2 to 2.7 mg N/L with accelerated denitrification efficiency (k: 0.04 to 0.27). However, 6.5 mg N/L of nitrite accumulated when the sulfide dosage exceeded 0.9 kg/m3/d (optimum value). The increasing electron export contribution of sulfide a maximum of 85.5% illustrated its competition with the in-situ sulfur. Meanwhile, over-dosing sulfide caused serious biofilm expulsion with significant decreases in the total biomass, live cell population, and ATP by 90.2%, 86.7%, and 54.8%, respectively. This study verified the capacity of dosing sulfide to improve the denitrification efficiency in S0PB but alerted the negative effect of exceeded dosing.

Recent Advances in Biotechnologies for the Treatment of Environmental Pollutants Based on Reactive Sulfur Species

The definition of reactive sulfur species (RSS) is inspired by the reactivity and variable chemical valence of sulfur. Sulfur is an essential element for life and is a part of global geochemical cycles. Wastewater treatment bioreactors can be divided into two major categories: sulfur reduction and sulfur oxidation. We review the origins of the definition of RSS and related biotechnological processes in environmental management. Sulfate reduction, sulfide oxidation, and sulfur-based redox reactions are key to driving the coupled global carbon, nitrogen, and sulfur co-cycles. This shows the coupling of the sulfur cycle with the carbon and nitrogen cycles and provides insights into the global material-chemical cycle. We also review the biological classification and RSS metabolic mechanisms of functional microorganisms involved in the biological processes, such as sulfate-reducing and sulfur-oxidizing bacteria. Developments in molecular biology and genomic technologies have allowed us to obtain detailed information on these bacteria. The importance of RSS in environmental technologies requires further consideration.

Insights into the Superior Bioavailability of Biogenic Sulfur from the View of Its Unique Properties: The Key Role of Trace Organic Substances

Elemental sulfur (S0) is widely utilized in environmental pollution control, while its low bioavailability has become a bottleneck for S0-based biotechnologies. Biogenic sulfur (bio-S0) has been demonstrated to have superior bioavailability, while little is known about its mechanisms thus far. This study investigated the bioavailability and relevant properties of bio-S0 based on the denitrifying activity of Thiobacillus denitrificans with chemical sulfur (chem-S0) as the control. It was found that the conversion rate and removal efficiency of nitrate in the bio-S0 system were 2.23 and 2.04 times those of the chem-S0 system. Bio-S0 was not pure orthorhombic sulfur [S: 96.88 ± 0.25% (w/w)]. Trace organic substances detected on the bio-S0 surface were revealed to contribute to its hydrophilicity, resulting in better dispersibility in the aqueous liquid. In addition, the adhesion force of T. denitrificans on bio-S0 was 1.54 times that of chem-S0, endowing a higher bacterial adhesion efficiency on the sulfur particle. The weaker intermolecular binding force due to the low crystallinity of bio-S0 led to enhanced cellular uptake by attached bacteria. The mechanisms for the superior bioavailability of bio-S0 were further proposed. This study provides a comprehensive view of the superior bioavailability of bio-S0 and is beneficial to developing high-quality sulfur resources.

Simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater by a bioelectrocatalysis coupled two-phase anaerobic reactor

The microbial electrolysis cell coupled the two-phase anaerobic digestion (MEC-TPAD) was developed for simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater. In acidogenic phase, the produced sulfides were efficiently converted into bio-sulfur via anodic bio-oxidation, with a maximum recovery of 59 ± 5.5 %. The anode coupled acidogenesis produced more volatile fatty acids which were benefit for the subsequent methanogenesis. The cathode in methanogenic phase created a suitable pH condition and enhanced the methanogenesis. Correspondingly, the maximum bio-methane yield in MEC-TPAD was 2 times higher than that in TPAD. Microbial communities revealed that major functional consortia capable of sulfides oxidation (e.g. Alcaligenes) in anode biofilm, hydrogenotrophic methanogenesis (e.g. Methanobacterium) in cathode biofilm, and acetotrophic methanogenesis (e.g. Methanosaeta) in methanogenic sludge were enriched. Economic benefit could totally cover the cost of input electric energy. This work opens an appealing avenue for recovering nutrient and energy from wastewater.

The mixed/mixotrophic nitrogen removal for the effective and sustainable treatment of wastewater: From treatment process to microbial mechanism

Biological nitrogen removal (BNR) is one of the most important environmental concerns in the field of wastewater treatment. The conventional BNR process based on heterotrophic nitrogen removal (HeNR) is suffering from several limitations, including external carbon source dependence, excessive sludge production, and greenhouse gas emissions. Through the mediation of autotrophic nitrogen removal (AuNR), mixed/mixotrophic nitrogen removal (MixNR) offers a viable solution to the optimization of the BNR process. Here, the recent advance and characteristics of MixNR process guided by sulfur-driven autotrophic denitrification (SDAD) and anammox are summarized in this review. Additionally, we discuss the functional microorganisms in different MixNR systems, shedding light on metabolic mechanisms and microbial interactions. The significance of MixNR for carbon reduction in the BNR process has also been noted. The knowledge gaps and the future research directions that may facilitate the practical application of the MixNR process are highlighted. Overall, the prospect of the MixNR process is attractive, and this review will provide guidance for the future implementation of MixNR process as well as deciphering the microbially metabolic mechanisms.

Weak electro-stimulation promotes microbial uranium removal: Efficacy and mechanisms

Removal and recovery of uranium from uranium-mine wastewater is beneficial to environmental protection and resource preservation. Reduction of soluble hexavalent U (U(VI)) to insoluble tetravalent uranium (U(IV)) by microbes is a plausible approach for this purpose, but its practical implementation has long been restricted by its intrinsic drawbacks. The electro-stimulated microbial process offers promise in overcoming these drawbacks. However, its applicability in real wastewater has not been evaluated yet, and its U(VI) removal mechanisms remain poorly understood. Herein, we report that introducing a weak electro-stimulation considerably boosted microbial U(VI) removal activities in both synthetic and real wastewater. The U(VI) removal has proceeded via U(VI)-to-U(IV) reduction in the biocathode, and the electrochemical characterization demonstrates the crucial role of the electroactive biofilm. Microbial community analysis shows that the broad biodiversity of the cathode biofilm is capable of U(VI) reduction, and the molecular ecological network indicates that synthetic metabolisms among electroactive and metal-reducing bacteria play major roles in electro-microbial-mediated uranium removal. Metagenomic sequencing elucidates that the electro-stimulated U(VI) bioreduction may proceed via e-pili, extracellular electron shuttles, periplasmic and outer membrane cytochrome, and thioredoxin pathways. These findings reveal the potential and mechanism of the electro-stimulated U(VI) bioreduction system for the treatment of U-bearing wastewater.

Coupled sulfur and electrode-driven autotrophic denitrification for significantly enhanced nitrate removal

Elemental sulfur (S⁰)-based autotrophic denitrification (SAD) has gained intensive attention in the treatment of secondary effluent for its low cost, high efficiency, and good stability. However, in practice, the supplementary addition of limestone is necessary to balance the alkalinity consumption during SAD operation, which increases water hardness and reduces the effective reaction volume. In this study, a coupled sulfur and electrode-driven autotrophic denitrification (SEAD) process was proposed with superior nitrate removal performance, less accumulation of sulfate, and self-balance of acidity-alkalinity capacity by regulating the applied voltage. The dual-channel electron supply from S⁰ and electrodes made the nitrate removal rate constant k in the SEAD process 3.7-5.1 and 1.4-3.5 times higher than that of the single electrode- and sulfur-driven systems, respectively. The S° contributed to 75.3%-83.1% of nitrate removal and the sulfate yield during SEAD (5.67-6.26 mg SO₄^2-/mg NO₃^--N) was decreased by 17%-25% compared with SAD. The S⁰ particle and electrode both as active bio-carriers constructed collaborative denitrification communities and functional genes. Pseudomonas, Ralstonia and Brevundimonas were the dominant denitrifying genera in S⁰ particle biofilm, while Pseudomonas, Chryseobacterium, Pantoea and Comamonas became dominant denitrifying genera in the cathode biofilm. The narG/Z/H/Y/I/V, nxrA/B, napA/B, nirS/K, norB/C and nosZ were potential functional genes for efficient nitrate reduction during the SEAD process. Metagenomic sequencing indicated that S⁰ as an electron donor has greater potential for complete denitrification than the electrode. These findings revealed the potential of SEAD for acting as a highly efficient post denitrification process.

The interaction between Pseudomonas C27 and Thiobacillus denitrificans in the integrated autotrophic and heterotrophic denitrification process

Compared to autotrophic and heterotrophic denitrification process, the integrated autotrophic and heterotrophic denitrification (IAHD) shows wider foreground of applications in the actual wastewaters with organic carbon, nitrogen and sulfur co-existing. The efficient co-removal of sulfur, nitrogen, and carbon in the IAHD system is guaranteed by the interaction between heterotrophic and autotrophic denitrificans. In order to further explore the interaction between functional bacteria, Pseudomonas C27 and Thiobacillus denitrifcans were selected as typical heterotrophic and autotrophic bacteria, and their characteristics metabolic responses to different sulfide concentrations were studied. Pseudomonas C27 had higher metabolic activity than T. denitrificans in the IAHD medium with sulfide concentration of 3.12-15.62 mmol/L. Moreover, the fastest sulfide removal rate (0.35 mmol/L·h) was achieved with a single inoculation of Pseudomonas C27. Meanwhile, in mixed inoculant conditions, the interaction between Pseudomonas C27 and T. denitrificans (P:T = 3:1, P:T = 1:1 and P:T = 1:3) yielded the highest sulfide removal efficiency (more than 85%) when sulfide concentration was 6.25-12.5 mmol/L. Additionally, the sulfide removal rate increased with the inoculation proportion of Pseudomonas C27. Thus, this apparent interaction provided a theoretical basis for further understanding and guidance on the efficient operation of IAHD system.

Effects of different carbon sources on the efficiency of sulfur-oxidizing denitrifying microorganisms

This study aims to compare the effects of different carbon sources on sulfur-oxidizing denitrifying microorganisms by using glucose, ethanol, and acetate as carbon sources. Under the same chemical oxygen demand Cr (COD_Cr), nitrate, and sulfide concentrations, the removal rate of nitrate and total organic carbon, and the yield of elemental sulfur in a static experiment and a continuous flow reactor with glucose as the carbon source were lower than those with ethanol and acetic acid as the carbon source. The core sulfur-oxidizing denitrifying bacteria that use glucose as the carbon source were Azoarcus, Geoalkalibacter, and Mangroviflexus; those that use ethanol as the carbon source were Arcobacter, Pseudomonas, and Thauera; those that use acetate as the carbon source were Pseudomonas and Azoarcus. The metabolic activity of microorganisms that use different carbon sources was explained by functional gene detection. The fluctuation of gltA, a functional gene indicating heterotrophic metabolism of microorganisms, was small in three reactors, but that of the sulfur oxidation gene, Sqr, in the reactor with acetic acid as the carbon source was larger. Our results suggest that acetate is a more suitable carbon source for denitrification-desulfurization systems.

Rapid start of high-concentration denitrification and desulfurization reactors by heterotrophic denitrification sulphur-oxidising bacteria

High sulphide concentrations can be toxic to denitrifying and desulphurising microorganisms. In this study, bioaugmentation was used to solve this problem. Pseudomonas sp. gs1 can tolerate 400 mg/L sulphide and converts most of the sulphide into elemental sulphur after 4 h. A solid inoculum of Pseudomonas sp. h1 was prepared. Two reactors, that is, one with and one without inoculum, were simultaneously run for 60 days. Bioreactor II to which bacterial inoculum was added reached a good treatment performance on day 3. The elemental sulphur concentration of the effluent was 342.6 mg/L. It was maintained at 245.3-333.8 mg/L during the subsequent operation. In contrast, reactor I without inoculants achieved the same performance on day 50. High-throughput sequencing shows that Pseudomonas and Azoarcus are the dominant genera. The abundance of the genus Pseudomonas and related denitrifying sulphur-oxidising bacteria in reactor I increases with the operation time. This phenomenon was confirmed by testing the sqr and gltA genes. The quantitative fluorescence PCR test also proves that the addition of bacteria leads to a rapid increase in the sulphur oxidation and carbon metabolism of the activated sludge in the reactor.

Facile preparation and characterization of a novel visible-light-responsive Rb2HgI4 nanostructure photocatalyst

Visible photocatalytic procedures exhibit encouraging potential in water purification by increasing the photocatalytic performance. Therefore, the improvement of low-cost and efficient photocatalysts for environmental remediation is an increasing demand, and photocatalysts based on semiconductors have gained considerable attention due to their superior stability and activity. In the current study, novel Rb₂HgI₄ nanostructures were prepared via a simple, low-cost, and low-temperature solid-state method. The effects of different parameters such as type of surfactants, reaction temperature, and reaction time were studied on the structure, crystallinity, particle size, and shape of nanostructures. This new compound has a suitable band gap (2.6 eV) in the visible region. The photocatalytic performance of Rb₂HgI₄ was examined for the removal of coloring agents under visible light irradiation and it was found that this compound could degrade and eliminate acid black 1 by about 72.1%.

Elemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment

Biochemical oxidation and reduction are the principle of biological water and wastewater treatment, in which electron donor and/or acceptor shall be provided. Elemental sulfur (S⁰) as a non-toxic and easily available material with low price, possesses both reductive and oxidative characteristics, suggesting that it is a suitable material for water and wastewater treatment. Recent advanced understanding of S⁰-respiring microorganisms and their metabolism further stimulated the development of S⁰-based technologies. As such, S⁰-based biotechnologies have emerged as cost-effective and attractive alternatives to conventional biological methods for water and wastewater treatment. For instance, S⁰-driven autotrophic denitrification substantially lower the operational cost for nitrogen removal from water and wastewater, compared to the conventional process with exogenous carbon source supplementation. The introduction of S⁰ can also avoid secondary pollution commonly caused by overdose of organic carbon. S⁰ reduction processes cost-effectively mineralize organic matter with low sludge production. Biological sulfide production using S⁰ as electron acceptor is also an attractive technology for metal-laden wastewater treatment, e.g. acid mine drainage. This paper outlines an overview of the fundamentals, characteristics and advances of the S⁰-based biotechnologies and highlights the functional S⁰-related microorganisms. In particular, the mechanisms of microorganisms accessing insoluble S⁰ and feasibility to improve S⁰ bio-utilization efficiency are critically discussed. Additionally, the research knowledge gaps, current process limitations, and required further developments are identified and discussed.

Simultaneous denitrification and desulfurization-S0 recovery of wastewater in trickling filters by bioaugmentation intervention based on avoiding collapse critical points

In order to better achieve efficiently simultaneous desulfurization and denitrification/S⁰ recovery of wastewater, the intervention of sulfur oxidizing bacteria (SOB) and denitrifying bacteria (DNB) was employed to avoid the collapse critical points (the dramatically decrease of S/N removal efficiency) under the fluctuated load. With the assistance of DNB and SOB, collapse critical point of trickling filter (TF) was delayed from the P₈ (105-114 d) to P₁₀ stage (129-138 d). The treatment efficiency of nitrogen and sulfur was the highest with the S/N ratio of 3:1. The bioaugmentation of DNB and SOB at collapse critical point could effectively regulated collapse situation, which further increased the maximum system utilization/elimination capacity to 4.50 kg S m^-3·h^-1 and 0.90 kg N m^-3·h^-1 (increased by 56.89% and 65.56% in comparison to control). High-throughput sequencing analysis indicated that Proteobacteria (average 78.59%) and Bacteroidetes (average 9.30%) were dominant bacteria in the reactor at all stages. As the reaction proceeds, the microbial community was gradually dominated by some functional genera such as Chryseobacterium (average 2.97%), Halothiobacillus (average 22.71%), Rhodanobacter (average 14.02%), Thiobacillus (average 9.01%), Thiomonas (average 16.70%) and Metallibacterium (average 21.63%), which could remove nitrate or sulfide. Both of Principal Component Analysis (PCA) and Canonical Correlation Analysis (CCA) demonstrated the important role of DNB/SOB during the long-term run in the trickling filters (TFs).

Production and characteristics of elemental sulfur during simultaneous nitrate and sulfide removal

The production and characteristics of elemental sulfur were examined during simultaneous sulfide and nitrate removal, with abiotic assays as control. The biotic assay showed good sulfide and nitrate removal, with the respective removal percentage of which were 90.67-96.88% and 100%. Nitrate reduction resulted in the production of nitrogen gas, while sulfate formed due to sulfide oxidation. The concentration of elemental sulfur in the effluent was greater than that in the sludge, which accounted for 73.70-86.28% of total elemental sulfur produced. Furthermore, the elemental sulfur of the effluent and sludge from the biotic assays was orthorhombic crystal S₈. Elemental sulfur was normally distributed in the effluent, but its average diameter increased with the increasing influent sulfide concentration (60-300 mg S/L), where the average diameter increased from 10 (60 mg S/L) to 29 μm (300 mg S/L).

Influence of microbial spatial distribution and activity in an EGSB reactor under high- and low-loading denitrification desulfurization

To characterize the impact of reactor configuration and influent loading on elemental sulphur (S⁰) recovery during denitrification desulfurization, a laboratory-scale expanded granular sludge bed (EGSB) reactor was established under two influent acetate/nitrate/sulphide loadings; the water flow velocity, microbial community, and functional genes at different heights were investigated. There was no S⁰ generated when acetate/nitrate/sulphide loadings were set to 0.95/0.60/1.05 kg/m³.d (low-loading). Furthermore, there were no typical denitrifying sulphide oxidizing bacteria under this condition, and Syntrophobacter, Anaerolineaceae genera were predominant in the reactor. As the influent loading was doubled (high-loading), S⁰ recovery increased to 87%; the bacterial distribution was relatively homogeneous with sulphide oxidation genera (Thauera) being predominant. Neither nirK nor sqr genes were detected in the low-loading sample at a height of 50 cm. The sqr/sox ratios of low-loading stage were 2.50 (10 cm), 0.94 (30 cm), and 0 (50 cm), and the ratios of the high-loading stage were 1.38 (10 cm), 1.33 (30 cm), and 1.08 (50 cm). A hydrodynamics analysis indicated that the water flow velocity was homogenous throughout the reactor. Appropriate reactor configuration and operation parameters play an important role in the efficient regulation of S⁰ recovery during denitrification desulfurization.

Short- and long-term effects of copper on anammox under gradually increased copper concentrations

This study aims to determine both short- and long-term response of enriched anammox culture to Cu. Assessment of short-term inhibition is based both on total applied Cu concentration and potential bioavailable fractions like intracellular, surface-bound, soluble and free Cu ion. The half maximal inhibitory concentration (IC50) values for total applied, soluble, intracellular and cell-associated concentrations were determined as 4.57 mg/L, 1.97 mg/L, 0.71 mg/L, 1.11 mg/L, respectively. Correlation between the surface-bound fraction of Cu and inhibition response was weak, suggesting that Cu sorbed to biomass was not directly responsible for the effects on anammox activity. There was a disparity between the results of short- and long-term experiments in terms of inhibition threshold concentration (i.e. short-term IC50 = 4.57 mg/L vs long-term IC50 = 6.74 mg/L). Candidatus Kuenenia (59.8%) and Candidatus Brocadia (40.2%) were the two main anammox genera within the initial biomass sample. One of the most interesting finding of the study is the demonstration that a complete wash-out of C. Brocadia genus at an applied Cu concentration of 6.5 mg/L. This strongly indicates that C. Brocadia were not able to tolerate high copper concentrations and all nitrogen conversion was carried out by C. Kuenenia during the Cu exposure period.

Relationship between functional bacteria in a denitrification desulfurization system under autotrophic, heterotrophic, and mixotrophic conditions

The denitrification desulfurization system can be used to remediate wastewater containing carbon, nitrogen, and sulfur. However, the relationship between autotrophic and heterotrophic bacteria remains poorly understood. To better understand the roles and relations of core bacteria, an expanded granular sludge bed (EGSB) reactor was continuously operated under autotrophic (stage I), heterotrophic (stage II) and mixotrophic (stages III-VII) conditions with a 490-day period. Stage IV represented the excellent S⁰ recovery rate (69.5%). The different trophic conditions caused the obvious succession of dominant bacterial genera. Autotrophic environment (stage I) enriched mostly Thiobacillus, and heterotrophic environment (stage II) was dominated with Azoarcus and Pseudomonas. Thauera, Arcobacter and Azoarcus became the predominant genera under mixotrophic conditions (stage III-VII). Strains belonged to these core genera were further isolated, and all seven isolates were confirmed with denitrifying sulfur oxidation capacity. Heterotrophic strain HDD1 (genus of Thauera) possessed both the highest sulfide degradation and S⁰ recovery rates. Expression levels of cbbM and gltA genes were positively related with the autotrophic and heterotrophic conditions, respectively. NirK gene was highly expressed between log 3.7-log 4.3 during the entire run. Expression of both sqr and soxB genes were closely related with sulfur conversion. More than 57.5% of S⁰ recovery rate could be obtained as sqr gene expression was greater than log 3.2, and while, sulfate was the primary form as soxB gene expression higher than log 3.9. The correlation between core microbial genera was very low from network, indicating a complex and non-specific mutualistic network between bacterial functional groups under each nutrient condition, and a stable coexistence state was possibly formed through utilizing each the secondary or waste metabolites in the mixotrophic conditions. This relationship was beneficial to the stability of the microbial community structure in the denitrification desulfurization system.

Spatial variation in bacterial biomass, community composition and driving factors across a eutrophic river

Eutrophication is a global problem, and bacterial diversity and community composition are usually affected by eutrophication. However, limited information on the ecological significance of bacterial community during algae blooms of rivers has been given, more studies should be focused on the bacterial diversity and distribution characteristics in eutrophic rivers. In this study, we explored the spatial variations of bacterial biomass, community structure, and their relationship with environmental factors in the eutrophic Xiangxi River. The content of Chlorophyll (Chl) was about 16 mg/L in the midstream (S2, S3), which was in the range of light eutrophication. Significant spatial variation of bacterial community structure was found at different sites and depths (p < 0.05), and the driving environmental factor was found to be nitrogen, mainly detected as total nitrogen (TN), Kjeldahl nitrogen (KN), and ammonia nitrogen (NH₄⁺) (p < 0.05). The midstream sites had some significantly different bacteria, including algicidal bacteria and dominant lineages during algal blooms. This result was consistent with the functional prediction, where significant higher abundance of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways was associated with algicidal substances in the midstream. At different water depths, some populations adapted to the surface layer, such as the class Flavobacteriia, and others preferred to inhabit in the bottom layer, such as Betaproteobacteria and Acidobacteria. The bacterial biomass was higher in the bottom layer than that in the surface and middle layer, and temperature and pH were found to be the major driving factors. The bacterial diversity increased with the increasing of depths in most sampling sites according to operational taxonomic units (OTUs), Chao1 and ACE indexes, and PO₄^3- was demonstrated to be the most significant factor. In summary, this study offered the evidence for microbial distribution characteristics across different sites and depths in summer, and its relationship with environmental variables in a eutrophic river.

Bioaugmentation with Thiobacillus sp. H1 in an autotrophic denitrification desulfurization microbial reactor: Microbial community changes and relationship

Thiobacillus sp. H1 was isolated and made into solid bacterial agent. The Thiobacillus sp. H1 agent was dosed into two reactor (all the agent dosed one-time, and multi-dosing bacteria evenly) and run for 40 days, a start-up with no microbial agent bioreactor as control. We found that the operational performance of multi-dosing inoculum reactor was stable, and the amount of elemental sulfur produced remained stable at 143.2-152.3 mg/L. The amount of elemental sulfur generated in the reactor without the addition of the inoculum was gradually increased, and the amount of elemental sulfur generated in the reactor with the inoculum added at one-time was decreased. Two kinds of Thiobacillus gen. and unclassified betaproteobacteria that coordinated the overall community function in the autotrophic denitrification desulfurization system with high-throughput sequencing. The trend of FccAB gene in each bioreactor was similar with the trend of elemental sulfur in the effluent. On the 5th day, the copy number of FccAB in bioreactor II was the highest among the three bioreactors, reaching 11.8 log copies L/g. This study explores the possibility of artificially synthesized denitrifying desulfurization flora in the future.

Succession of functional bacteria in a denitrification desulphurisation system under mixotrophic conditions

Large-scale use of ammonia, sulphate, and nitrate in industrial manufacturing has resulted in the generation of industrial wastewater pollutants. However, approaches to eliminate such contamination have not been extensively studied. Accordingly, in this study, we investigated the succession of bacteria under different influent loadings in a mixotrophic denitrification desulphurisation system. Four expanded granular sludge bed reactors were operated simultaneously. The sulphide loading of reactor I was 1.2 kg/m³‧day, the sulphide load of reactor II was 2.4 kg/m³‧day, and the sulphide load of reactor III was 3.6 kg/m³‧day. The molar ratio of carbon versus nitrogen in the influent under each condition was fixed at 1.26:1, and the molar ratio of sulphur versus nitrogen was fixed at 5:6; each reactor was operated for 90 days. Reactor IV was a verification reactor. The three conditions were repeated, and each condition was operated for 90 days. Middle- and late-stage samples under each condition were sequenced using a high-throughput sequencer. Azoarcus, Thauera, Arcobacter, and Pseudomonas were the core genera of the denitrification desulphurisation system under mixotrophic conditions. The genus Azoarcus was a cornerstone genus of mixotrophic conditions, as demonstrated using the random forest model and correlation network analysis.

The stimulating metabolic mechanisms response to sulfide and oxygen in typical heterotrophic sulfide-oxidizing nitrate-reducing bacteria Pseudomonas C27

Micro-aeration is an effective tool that helps integrated autotrophic and heterotrophic denitrification process to withstand high sulfide concentration by making heterotrophic sulfide-oxidizing nitrate-reducing bacteria (h-soNRB) prevail. For further understanding of the dominance of h-soNRB, Pseudomonas C27 was selected as the typical bacterium and its metabolic characteristics responding to sulfide and oxygen stimulation were studied. Under high sulfide concentration condition, addition of trace oxygen led to a two-stage sulfide oxidation process, and sulfide oxidation rate in the first stage was 1.4 times more than that under anaerobic condition. According to transcriptome analysis, the pdo gene significantly up-regulated 2.36 and 2.57 times with and without oxygen under stimulation of high sulfide concentration. Additionally, two possible enhanced sulfide removal pathways coping with high sulfide concentration, namely sqr-cysI-gpx-gor-glpE and cysK-gshA-gshB-pdo-glpE, caused by oxygen were proposed in Pseudomonas C27. These findings provide a theoretical basis for locating high-efficiency sulfur oxidase in h-soNRB.

Heterotrophic sulfide-oxidizing nitrate-reducing bacteria enables the high performance of integrated autotrophic-heterotrophic denitrification (IAHD) process under high sulfide loading

Micro-aerobic enhancement technology has been developed as an effective tool to enhance simultaneous removal of sulfide, nitrate and organic carbon during the integrated autotrophic-heterotrophic denitrification (IAHD) process under high loading; however, its mechanism of enhancement for functional bacteria remains ambiguous. In this study, we discovered that heterotrophic sulfide-oxidizing nitrate-reducing bacteria (h-soNRB) are responsible for enhancing IAHD performance under micro-aerobic conditions with high sulfide loading. In a continuous IAHD bioreactor, aeration rate of 2.6 mL min^-1·L^-1 promoted 2 to 4 times higher removal efficiencies of sulfide, nitrate and acetate with an influent sulfide concentration of 18.75 mmol/L. Metagenomic analysis revealed that trace oxygen stimulated the abundance of genes responsible for sulfide oxidation (sqr, glpE, pdo, sox and cysK), which were upregulated by 15.2%-129.9%, and the genes encoding nitrate reductase were up-regulated by 67.4%. The increased acetate removal efficiency was attributed to upregulation of ack, pta and TCA cycle related genes. The h-NRB Pseudomonas, Azoarcus, Thauera and Halomonas were detected and regarded as h-soNRB in our bioreactor. According to Illumina MiSeq sequencing, these genera were absolutely dominant in the micro-aerobic microbial community at relative abundances ranging from 82.72% to 90.84%. The sulfide, nitrate and acetate removal rates of Pseudomonas C27, a typical h-soNRB, were at least 10 times higher under micro-aerobic conditions than under anaerobic conditions. Besides, the sulfur, nitrogen and carbon metabolic network was constructed based on the Pseudomonas C27 genome. The pdo and cysK genes found in this strain may be the most advantageous for autotrophic sulfide oxidizing nitrate reducing bacteria (a-soNRB), which are closely related to the high-efficiency sulfide, nitrate and acetate removal performance under high sulfide concentrations and a limited oxygen supply. In addition, after micro-aerobic cultivation, the anaerobic sulfide loading tolerance of the IAHD bioreactor increased from 18.75 to 37.5 mmol/L with sulfide, nitrate and acetate removal efficiencies increasing 1.5 to 3 times, which suggests that intermittent micro-aeration might be a more economical and efficient regime for high-sulfide IAHD regulation.

Potential applications of endogenous sulfide for enhanced denitrification of low C/N domestic wastewater in anodic mixotrophic denitrification microbial fuel cell: The mechanism of electrons transfer and microbial community

Anodic mixotrophic denitrification microbial fuel cell (MFC) was developed for pollutants removal and electricity generation in treatment of low C/N domestic wastewater. The experimental results show that the MFC achieved up to 100% of acetate, 100% of sulfide, and more than 91% of nitrate removal efficiency in all the MFCs. Particularly, thiosulfate was generated as the main intermediate of sulfide oxidation, and the sulfate generation ratio ranged from 66.93% to 73.76%. Those electrons produced during the acetate and sulfide oxidation were mainly used for denitrification and electricity generation. The microbial community analysis revealed that heterotrophic denitrifying bacteria (HDB) and sulfide-based autotrophic denitrifying bacteria (SADB) were the dominant bacteria for pollutants removal, and those facultative autotrophic bacterium (FAB) were key functional genera for high sulfate generation under both low and high sulfide concentrations. Meanwhile, the microbial functional prediction revealed that sulfide oxidation gene of Sqr and Sox were highly expressed. Moreover, a preliminary sulfide-based autotrophic denitrification (SAD) potential estimation indicated that the sulfide generated in the WWTPs had great potential for denitrification.

Enhanced methane production by alleviating sulfide inhibition with a microbial electrolysis coupled anaerobic digestion reactor

Anaerobic digestion (AD) of organics is a challenging task under high-strength sulfate (SO₄^2-) conditions. The generation of toxic sulfides by SO₄^2--reducing bacteria (SRB) causes low methane (CH₄) production. This study investigated the feasibility of alleviating sulfide inhibition and enhancing CH₄ production by using an anaerobic reactor with built-in microbial electrolysis cell (MEC), namely ME-AD reactor. Compared to AD reactor, unionized H₂S in the ME-AD reactor was sufficiently converted into ionized HS^- due to the weak alkaline condition created via cathodic H₂ production, which relieved the toxicity of unionized H₂S to methanogenesis. Correspondingly, the CH₄ production in the ME-AD system was 1.56 times higher than that in the AD reactor with alkaline-pH control and 3.03 times higher than that in the AD reactors (no external voltage and no electrodes) without alkaline-pH control. MEC increased the amount of substrates available for CH₄-producing bacteria (MPB) to generate more CH₄. Microbial community analysis indicated that hydrogentrophic MPB (e.g. Methanosphaera) and acetotrophic MPB (e.g. Methanosaeta) participated in the two major pathways of CH₄ formation were successfully enriched in the cathode biofilm and suspended sludge of the ME-AD system. Economic revenue from increased CH₄ production totally covered the cost of input electricity. Integration of MEC with AD could be an attractive technology to alleviate sulfide inhibition and enhance CH₄ production from AD of organics under SO₄^2--rich condition.

Recirculation ratio regulates denitrifying sulfide removal and elemental sulfur recovery by altering sludge characteristics and microbial community composition in an EGSB reactor

Expanded granular sludge blanket (EGSB) is regarded as a promising reactor to carry out denitrifying sulfide removal (DSR) and elemental sulfur (S⁰) recovery. Although the recirculation ratio is an essential parameter for EGSB reactors, how it impacts the DSR process remains poorly understood. Here, three lab-scale DSR-EGSB reactors were established with the different recirculation ratios (3:1, 6:1 and 9:1) to evaluate the corresponding variations in pollutant removal, S⁰ recovery, anaerobic granular sludge (AGS) characteristics and microbial community composition. It was found that an intermediate recirculation ratio (6:1) could facilitate long-term reactor stability. Adequate recirculation ratio could enhance S⁰ recovery, but an excessive recirculation ratio (9:1) was likely to cause AGS fragmentation and biomass loss. The S⁰ desorbed more from sludge at higher recirculation ratios, probably due to the enhanced hydraulic disturbance caused by the increased recirculation ratios. At the low recirculation ratio (3:1), S⁰ accumulation as inorganic suspended solids in AGS led to a decrease in VSS/TSS ratio and mass transfer efficiency. Although typical denitrifying and sulfide-oxidizing bacteria (e.g., Azoarcus, Thauera and Arcobacter) were predominant in all conditions, facultative and heterotrophic functional bacteria (e.g., Azoarcus and Thauera) were more adaptable to higher recirculation ratios than autotrophs (e.g., Arcobacter, Thiobacillus and Vulcanibacillus), which was conducive to the formation of bacterial aggregates to response to the increased recirculation ratio. The study revealed recirculation ratio regulation significantly impacted the DSR-EGSB reactor performance by altering AGS characteristics and microbial community composition, which provides a novel strategy to improve DSR performance and S⁰ recovery.

Continuous sulfur biotransformation in an anaerobic-anoxic sequential batch reactor involving sulfate reduction and denitrifying sulfide oxidization

The pathways and intermediates of continuous sulfur biotransformation in an anaerobic and anoxic sequential batch reactor (AA-SBR) involving sulfate reduction (SR) and denitrifying sulfide oxidization (DSO) were investigated. In the anoxic phase, DSO occurred in two sequential steps, the oxidation of sulfide (S^2-) to elemental sulfur (S⁰) and the oxidation of S⁰ to sulfate (SO₄^2-). The oxidation rate of S^2- to S⁰ was 3.31 times faster than that of S⁰ to SO₄^2-, resulting in the accumulation of S⁰ as a desired intermediate under S^2--S/NO₃^--N ratio (molar ratio) of 0.9:1. Although, approximately 60% of generated S⁰ suspended in the effluent, about 40% of S⁰ retained in the sludge, which could be further oxidized or reduced in anoxic or anaerobic phase. In anoxic, S⁰ was subsequently oxidized to SO₄^2- under S^2--S/NO₃^--N ratio of 0.5:1. In anaerobic, S⁰ coexist with SO₄^2- (in fresh wastewater) were simultaneously reduced to S^2-, and the reduction rate of SO₄^2- to S^2- was 3.17 times faster than that of S⁰ to S^2-, resulting in a higher production of S⁰ in subsequent anoxic phase. Microbial community analysis indicated that SO₄^2-/S⁰-reducing bacteria (e.g. Desulfomicrobium and Desulfuromonas) and S^2-/S⁰-oxidizing bacteria (e.g. Paracoccus and Thermothrix) co-participated in continuous sulfur biotransformation in the AA-SBR. A conceptual model was established to describe these main processes and key intermediates. The research offers a new insight into the reaction processes optimization for S⁰ recovery and simultaneous removal of SO₄^2- and NO₃^- in an AA-SBR.

Effect of nitrite and nitrate on sulfate reducing ammonium oxidation

The effects of nitrite and nitrate on the integration of ammonium oxidization and sulfate reduction were investigated in a self-designed reactor with an effective volume of 5 L. An experimental study indicated that the ammonium oxidization and sulfate reduction efficiencies were increased in the presence of nitrite and nitrate. Studies showed that a decreasing proportion of N/S in the presence of NO₂ ^- at 30 mg·L^-1 would lead to high removal efficiencies of NH₄ ⁺-N and SO₄ ^2--S of up to 78.13% and 46.72%, respectively. On the other hand, NO₃ ^- was produced at approximately 26.89 mg·L^-1. Proteobacteria, Chloroflexi, Bacteroidetes, Chlorobi, Acidobacteria, Planctomycetes and Nitrospirae were detected in the anaerobic cycle growth reactor. Proteobacteria was identified as the dominant functional bacteria removing nitrogen in the reactor. The nitritation reaction could promote the sulfate-reducing ammonium oxidation (SRAO) process. NH₄ ⁺ was converted to NO₂ and other intermediates, for which the electron acceptor was SO₄ ^2-. These results showed that nitrogen was converted by the nitrification process, the denitrification process, and the traditional anammox process simultaneously with the SRAO process. The sulfur-based autotrophic denitration and denitrification in the reactor were caused by the influent nitrite and nitrate.

Efficient reduction of nitrobenzene by sulfate-reducer enriched biocathode in microbial electrolysis cell

This study aimed to enhance treatment of wastewater containing nitrobenzene (NB) and sulfate using biocathode enriched with sulfate-reducing bacteria in microbial electrolysis cell (MEC). Artificial wastewater with 50 mg L^-1 NB and 200 mg L^-1 sulfate was used as the catholyte. With 0.8 V applied voltage, removal efficiencies of NB and sulfate reached 98% and 34%, respectively, within 36 h. Aniline and sulfide were the main reductive products in the catholyte with concentrations increased to 0.32 and 0.51 mM, which accounted for 97% and 78% of the removed NB and sulfate, respectively. Sulfate-reducer Desulfovibrio sp. and Wolinella sp. played the dominant role in the biocathode for the reductions of sulfate and NB. Analyses of scanning electron microscope and X-ray photoelectron spectroscopy showed the formation of elemental S on the biocathode surface. The relative abundance of sulfur-oxidizing bacterium Thioclava sp. reached 24% on the biocathode. The results indicated that the oxidation process from S^2- to S⁰ occurred on the biocathode, which provided electrons to biofilm for NB reduction. The MEC with sulfate-reducer enriched biocathode can be used as an alternative to treat complex wastewater containing NB and sulfate.

Identification of biofilm formation and exoelectrogenic population structure and function with graphene/polyanliline modified anode in microbial fuel cell

Improving anode configuration with polymer or nanomaterial modification is promising for enhancing microbial fuel cell performance. However, how anode modification affects biofilm development and electrogenic function remains poorly understood. In this study, the carbon cloth anode modified with polyaniline and reduced graphene oxide was successfully fabricated which obtained the highest power output. Accelerated electrogenic biofilm formation and the better electrogenic bacterial colonization based on the superior material properties (preferable electrochemical characteristics, the film-like structure and the more activated sites) were observed with the in situ biofilm development monitoring. The acclimation time was 2.4 times shorter with graphene and polyaniline modified anode than the bare one when inoculated with wastewater. Biofilm structure and function analysis show that Geobacter is the most predominant with the abundance as high as 81.4%, and meanwhile, electrogenesis related outer-surface octaheme c-type cytochrome omcZ is highly expressed in the modified anode. The anode modified with graphene and polyaniline favors Geobacter colonization, accelerates electrogenic biofilm formation and improves omcZ expression level, eventually leading to the improved performance of microbial fuel cell. The study for the first time reveals the impacts on biofilm development and microbial function by anode modification, which will better guide the potential application of microbial fuel cell for wastewater recovery.

Bioreactor performance and microbial community analysis of autotrophic denitrification under micro-aerobic condition

Autotrophic denitrification process is an effective strategy for treating sulfide and nitrate-enriched wastewater with low organic carbon. This study determined the sulfide oxidation and nitrate reduction rates and characterized the dominant bacteria and microbial community structure stimulated by micro-aerobic conditions in autotrophic denitrification system. With gradually increased sulfide concentration, the sulfide removal rate decreased to 37.8% at S^2- = 600 mg/L, while the peak sulfide and nitrate removal rates (100% and 53.8%) were achieved at S^2- = 800 mg/L with the air aeration rate of 20 mL/min. The Illumina sequencing results indicated that Thiobacillus accounted for 63% of total operational taxonomic units at generic level with sulfide concentration of 200 mg/L under anaerobic condition. However, Azoarcus, Thauera and Aliidiomorina became the dominant genera under micro-aerobic condition and their abundance significantly and positively related to the sulfide concentration and aeration rate (p < 0.05). According to the 16S metaproteomics functional composition prediction, one potential mechanism for autotrophic denitrifying under micro-aerobic condition was deduced that the oxidation of sulfide to thiosulfate further to sulfite was reinforced by trace oxygen, while the sulfite reductase activity was restrained. The decreased sulfide concentration weakened the toxicity inhibition on denitrifiers and accordingly the performance of autotrophic denitrification process was enhanced by micro-aerobic condition.

Effect of quorum quenching on biofouling and ammonia removal in membrane bioreactor under stressful conditions

Quorum quenching (QQ) has been used to control biofouling in membrane bioreactors (MBRs), but the effect of QQ on the performance of MBR has not been systematically studied. This study investigated the effect of QQ on ammonia removal in MBR especially in some stressful conditions. The results showed that membrane fouling was effectively alleviated by QQ in all conditions. For the short HRT (3.94 h), the ammonia removal in QQ-MBR was fluctuating. In the presence of nitrification inhibitors (acetonitrile and allylthiourea) or at low temperature (10 °C), QQ induced much more significant suppression on nitrification in batch test and MBR. The number of the ammonia oxidizing bacteria (AOB) was not decreasing in these situations, which indicated that QQ only suppressed the activity of AOB. In all, comprehensive considerations should be taken into account when applying a QS tuning strategy to a bioreactor.