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

Effect of different carbon sources on sulfate reduction and microbial community structure in bioelectrochemical systems

Microbial electrolysis cells (MECs) have rapidly developed into a promising technology to treat sulfate-rich wastewater that lacks electron donors. Hence, a better understanding of the effect on the microbial community structure caused by different sources in bioelectrochemical systems is required. This study sought to investigate the effect of different carbon sources (NaHCO₃, ethanol, and acetate were employed as sole carbon source respectively) on the performance of sulfate-reducing biocathodes. The sulfate reduction efficiency enhanced by the bioelectrochemical systems was 8.09 - 11.57% higher than that of open-circuit reference experiments. Furthermore, the optimum carbon source was ethanol with a maximum sulfate reduction rate of 170 mg L^-1 d^-1 in the bioelectrochemical systems. The different carbon sources induced significant differences in sulfate reduction efficiency as demonstrated by the application of a micro-electrical field. Microbial community structure and network analysis revealed that all three kinds of carbon source systems enriched large proportions of sulfate-reducing bacteria and electroactive bacteria but were significantly distinct in composition. The dominant sulfate-reducing bacteria that use NaHCO₃ and acetate as carbon sources were Desulfobacter and Desulfobulbus, whereas those that use ethanol as carbon source were Desulfomicrobium and Desulfovibrio. Our results suggest that ethanol is a more suitable carbon source for sulfate reduction in bioelectrochemical systems.

Roles, mechanism of action, and potential applications of sulfur-oxidizing bacteria for environmental bioremediation

Sulfur (S) is a crucial component in the environment and living organisms. This work is the first attempt to provide an overview and critical discussion on the roles, mechanisms, and environmental applications of sulfur-oxidizing bacteria (SOB). The findings reveal that key enzymes of SOB embarked on oxidation of sulfide, sulfite, thiosulfate, and elemental S. Conversion of reduced S compounds was oxidatively catalyzed by various enzymes (e.g. sulfide: quinone oxidoreductase, flavocytochrome c-sulfide dehydrogenase, dissimilatory sulfite reductase, heterodisulfide reductase-like proteins). Environmental applications of SOB discussed include detoxifying hydrogen sulfide, soil bioremediation, and wastewater treatment. SOB producing S⁰ engaged in biological S soil amendments (e.g. saline-alkali soil remediation, the oxidation of sulfide-bearing minerals). Biotreatment of H₂S using SOB occurred under both aerobic and anaerobic conditions. Sulfide, nitrate, and sulfamethoxazole were removed through SOB suspension cultures and S⁰-based carriers. Finally, this work presented future perspectives on SOB development, including S⁰ recovery, SOB enrichment, field measurement and identification of sulfur compounds, and the development of mathematical simulation.

Metagenomic analysis of microbial community and metabolic pathway of simultaneous sulfide and nitrite removal process exposed to divergent hydraulic retention times

The role of hydraulic retention time (HRT) on S⁰ production was assessed through metagenomics analyses. Considering comprehensive performance for the tested HRTs (0.25-13.33 h), the optimal HRT was 1 h, while respective sulfide and nitrite loading rate could reach 6.84 kg S/(m³·d) and 1.95 kg N/(m³·d), and total S⁰ yield was 0.36 kg S/(kg (VSS)·d). Bacterial community richness decreased along the shortening of HRT. Microbacterium, Sulfurimonas, Sulfurovum, Paracoccus and Thauera were highly abundant bacteria. During sulfur metabolism, high expression of sqr gene was the main reason of maintaining high desulfurization load, while lacking soxB caused the continuous increase of S⁰. Regarding nitrogen metabolism, the rapid decrease of nitrite transporter prevented nitrite to enter in cells, which caused a rapid decrease of nitrite removal under extreme HRT. Adjusting HRT is an effective way to enhance S⁰ production for the application of the simultaneous sulfide and nitrite removal process.

Regulating intermediates to realize the coupled hydrion with biology polysulfide in wastewater treatment

The purpose of this study is to clarify the mechanism of the coupled hydrion with biology polysulfide in the simultaneous denitrification and desulfurization process. The coupled hydrion with biology polysulfide, uncoupled hydrion with biology polysulfide and no polysulfide experiments were performed in wastewater with two kinds of sulfide loads (100 and 200 mg/L). When the concentration of thiosulfate was suitable, the free H⁺ concentration (74.2 and 91.0 mg/L) and the proportion of Thiobacillus denitrificans (85.4% and 59.7%) were both higher under the two kinds of sulfide loading conditions (100 and 200 mg/L), and coupled hydrion with biology polysulfide was realized (the production of elemental sulfur is as high as 33 and 101 mg/L). Further analysis shown that the way of coupled hydrion with biology polysulfide were both: 2.0S2-+6.4NO3-+30.1H++21.7e-→1.0S2-+1.0SO42-+3.2N2+15.0H2O. In addition, for the coupled hydrion with biology polysulfide, more nitrates could be utilized to produce elemental sulfur S⁰, and the lower ratio of H⁺/S⁰ and SO42-/S0 were observed (S^2- = 100 mg/L: 2.3 and 0.9; S^2- = 200 mg/L: 0.9 and 0.03), which could promote the growth of Thiobacillus denitrificans and increase the proportion of Thiobacillus denitrificans. This maybe one of the reasons why coupled hydrion with biology polysulfide could be achieved.

Sulfur transformation and bacterial community dynamics in both desulfurization-denitrification biofilm and suspended activated sludge

Types of microbial aggregates have essential effects on bacterial communities' characteristics, thus affecting the pollutants removal. An up-flow biofilm reactor was used to study the different performances of S^2-/NO₂^- removal and functional genes in suspended sludge and biofilms. The metabolic pathways of sulfurous and nitrogenous pollutants in the desulfurization-denitrification process were proposed. The results showed that S⁰ formation dominated the reactor with a high S^2- concentration. Autotrophic Sulfurovum responsible for S^2-/S⁰ oxidation was the only dominant bacteria in suspended sludge. Heterotrophic Desulfocapsa responsible for SO₄^2- reduction coexisted with Sulfurovum and dominated in biofilms. S^2- oxidation to S⁰ was catalyzed via fccA/B and sqr genes in suspended sludge. S₃^2-/S⁰ oxidation to SO₄^2- was catalyzed via dsrA/B gene in biofilms. SO₄^2- and NO₂^- were removed via the dissimilatory sulfate reduction and denitrification pathway, respectively. This work provides a fundamental and practical basis for optimizing suspended sludge/biofilm systems for S^2-/NO₂^- removal.

Effects of salinity shock on simultaneous nitrification and denitrification by a membrane bioreactor: Performance, sludge activity, and functional microflora

Physical and chemical treatments of Tungsten smelting wastewater, with high salt content and low C/N ratio, are often tedious. As a solution, this study suggested a simultaneous nitrification and denitrification membrane bioreactor (SND-MBR) for salinity gradient domestication. During the salinity acclimation period, we observed 20% and 11% removal of NH₄⁺-N and Chemical Oxygen Demand (COD), respectively. However, the SND efficiency reached 95.55% after stable operation at 3.0% salinity. Through stoichiometric and kinetic analyses, we confirmed that increased salinity significantly inhibited electron transport system activity, nitrification, and denitrification, evidenced by the extremely low ammonia monooxygenase and nitrite reductase activities. Further high-throughput sequencing showed that Nitrosomonas dominated the functional microbial flora succession and denitrification in high salinity environments. In comparison with a control, the Kyoto Encyclopedia of Genes and Genomes analysis showed that wastewater salinity weakened the functional gene level of MBR microbial flora, and the enzyme key to the assimilation nitrate reduction changed from nitrate reductase to assimilation nitrate reductase.

Microbial interaction promotes desulfurization efficiency under high pH condition

The existence of H₂S in biogas may cause equipment corrosion and considerable SO₂ emission. Commonly used biotrickling filters may cause biogas dilution or generation of explosive mixtures. Compared with biotrickling filters, two-step process such as bioscrubber filters can overcome these shortages. However, its removal efficiency was still limited due to low microbial activity under high pH condition. Here, a bioreactor filter was carried out under pH 9.0. Removal efficiency higher than 99% was achieved under sulfide loading rate reaching 4.24 kg S m^-3d^-1. Results of network and high throughput sequencing showed that Thiobacillus acted as both dominant species (accounting for 75%) and unique kinless hub in this bioreactor. Other bacteria (accounting for 25%) contributed 75% to the network, which implied the intensive interaction between Thiobacillus and others. Sulfide removal ability and pH tolerance of pure bacteria and mixed culture were considered to verify how microbial interaction influenced them. Compared with pure bacteria, mixed culture had better performance under high pH condition, which confirmed that microbial interaction promoted desulfurization efficiency under high pH condition. These results showed that intensive microbial interaction might be the key to enhance sulfide removal efficiency under high pH condition.

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).

Long-term stable and energy-neutral mixed biofilm electrode for complete nitrogen removal from high-salinity wastewater: Mechanism and microbial community

The steady mixed biofilm electrode (MBE) was investigated for the removal of nitrogen from mustard tuber wastewater. Results showed that complete nitrogen removal occurred over a wide initial chemical oxygen demand (COD)/total nitrogen (TN) ratio ranging from 2.8 to 9.8 using MBE. MBE revealed broad-spectrum applicability for the treatment of high-salinity wastewater containing different forms of nitrogen. Bio-electrochemical process, in-situ heterotrophic nitrogen reduction, ammonia stripping, nitrogen assimilation, and endogenous denitrification coexisted for the removal of nitrogen. Batch activity tests and functional microorganism analysis confirmed that autotrophic/heterotrophic nitrification, anoxic/aerobic denitrification, and nitrogen bio-electrochemical reduction cooperated to achieve efficient nitrogen conversion. More importantly, the analysis of the preliminary energy balance demonstrated that MBE was self-sustaining. The long-term operation stability of MBE was of great importance for its practical application. The results provided herein offer new insights into bioelectrochemical nitrogen removal and resource treatment of high-salinity wastewater.

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.