Pin-point denitrification for groundwater purification without direct chemical dosing: Demonstration of a two-chamber sulfide-driven denitrifying microbial electrochemical system

Biogenic sulfur recovery from sulfate-laden antibiotic production wastewater using a single-chamber up-flow bioelectrochemical reactor

The high-concentration sulfate (SO42-) in the antibiotic production wastewater hinders the anerobic methanogenic process and also proposes possible environmental risk. In this study, a novel single-chamber up-flow anaerobic bioelectrochemical reactor (UBER) was designed to realize simultaneous SO42- removal and elemental sulfur (S0) recovery. With the carbon felt, the cathode was installed underneath and the anode above to meet the different biological niches for sulfate reducing bacteria (SRB) and sulfur oxidizing bacteria (SOB). The bio-anode UBER (B-UBER) demonstrated a much higher average SO42- removal rate (SRR) of 113.2 ± 5.7 mg SO42--S L-1 d-1 coupled with a S0 production rate (SPR) of 54.4 ± 5.8 mg S0-S L-1 d-1 at the optimal voltage of 0.8 V than that in the abio-anode UBER (control reactor) (SRR = 86.6 ± 13.4 mg SO42--S L-1 d-1; SPR = 25.5 ± 9.7 mg S0-S L-1 d-1) under long-term operation. A large amount of biogenic S0 (about 72.2 mg g-1 VSS) was recovered in the B-UBER. The bio-anode, dominated by Thiovirga (SOB genus) and Acinetobacter (electrochemically active bacteria genus), exhibited a higher current density, lower overpotential, and lower internal resistance. C-type cytochromes mainly served as the crucial electron transfer mediator for both direct and indirect electron transfer, so that significantly increasing electron transfer capacity and biogenic S0 recovery. The reaction pathways of the sulfur transformation in the B-UBER were hypothesized that SRB utilized acetate as the main electron donor for SO42- reduction in the cathode zone and SOB transferred electrons to the anode or oxygen to produce biogenic S0 in the anode zone. This study proved a new pathway for biogenic S0 recovery and sulfate removal from sulfate-laden antibiotic production wastewater using a well-designed single-chamber bioelectrochemical reactor.

Sulfur bacteria-reinforced microbial electrochemical denitrification

Two limiting factors of microbial electrochemical denitrification (MED) are the abundance and efficiency of the functional microorganisms. To supply these microorganisms, MED systems are inoculated with denitrifying sludge, but such method has much room for improvement. This study compared MED inoculated with autotrophic denitrifying inoculum (ADI) versus with heterotrophic denitrifying inoculum (HDI). ADI exhibited electroactivity for 50% less of timethan HDI. The denitrification efficiency of the ADI biocathode was42% higherthan that of the HDI biocathode. The HDI biocathode had high levels of polysaccharides while the ADI biocathode was rich in proteins, suggesting that two biocathodes may achieveMED but via differentpathways. Microbial communities of two biocathodes indicated MED of HDI biocathode may rely on interspecies electron transfer, whereas sulfur bacteria of ADI biocathode take electrons directly from the cathode to achieve MED. Utilizing autotrophic sulfur-oxidizing denitrifiers, this study offers a strategy for enhancing MED.

Enhancing biocathode denitrification performance with nano-Fe3O4 under polarity period reversal

The presence of excessive concentrations of nitrate poses a threat to both the environment and human health, and the bioelectrochemical systems (BESs) are attractive green technologies for nitrate removal. However, the denitrification efficiency in the BESs is still limited by slow biofilm formation and nitrate removal. In this work, we demonstrate the efficacy of novel combination of magnetite nanoparticles (nano-Fe3O4) with the anode-cathode polarity period reversal (PPR-Fe3O4) for improving the performance of BESs. After only two-week cultivation, the highest cathodic current density (7.71 ± 1.01 A m-2) and NO3--N removal rate (8.19 ± 0.97 g m-2 d-1) reported to date were obtained in the PPR-Fe3O4 process (i.e., polarity period reversal with nano-Fe3O4 added) at applied working voltage of -0.2 and -0.5 V (vs Ag/AgCl) under bioanodic and biocathodic conditions, respectively. Compared with the polarity reversal once only process, the PPR process (i.e., polarity period reversal in the absence of nano-Fe3O4) enhanced bioelectroactivity through increasing biofilm biomass and altering microbial community structure. Nano-Fe3O4 could enhance extracellular electron transfer as a result of promoting the formation of extracellular polymers containing Fe3O4 and reducing charge transfer resistance of bioelectrodes. This work develops a novel biocathode denitrification strategy to achieve efficient nitrate removal after rapid cultivation.

Optimization of the chemolithotrophic denitrification of ion exchange concentrate using hydrogen-based membrane biofilm reactors

A H2-based membrane biofilm reactor (MBfR) was used to remove nitrate from a synthetic ion-exchange brine made up of 23.8 g L-1 NaCl. To aid the selection of the best nitrate management strategy, our research was based on the integrated analysis of ionic exchange and MBfR processes, including a detailed cost analysis. The nitrate removal flux was not affected if key nutrients were present in the feed solution including potassium and sodium bicarbonate. Operating pH was maintained between 7 and 8. By using a H2 pressure of 15 psi, a hydraulic retention time (HRT) of 4 h, and a surface loading rate of 13.6 ± 0.2 g N m-2 d-1, the average nitrate removal flux was 3.3 ± 0.6 g N m-2 d-1. At HRTs of up to 24 h, the system was able to maintain a removal flux of 1.6 ± 0.2 g N m-2 d-1. Microbial diversity analysis showed that the consortium was dominated by the genera Sulfurimonas and Marinobacter. The estimated cost for a 200 m3/h capacity, coupled ion exchange (IX) + MBfR treatment plant is 0.43 USD/m3. This is a sustainable and competitive alternative to an IX-only plant for the same flowrate. The proposed treatment option allows for brine recycling and reduces costs by 55% by avoiding brine disposal expenses.

Desulfurization of hydrophilic and hydrophobic volatile reduced sulfur with elemental sulfur production in denitrifying bioscrubber

Volatile reduced sulfur compounds were odor and irritating toxic gas, which were commonly produced during waste and wastewater treatment. The autotrophic sulfide denitrifiers converted sulfide as alternative electron acceptor to reduce nitrate, which achieved simultaneous denitrification and sulfur oxidation. In this study, to investigate the effect of sulfur compounds solubility, S/N and oxygen on sulfur and nitrogen removal, a bioscrubber was studied for treatment of hydrophilic H₂S and hydrophobic CS₂. Both H₂S and CS₂ could be efficiently removed (99%), with the highest sulfide loading of 46.9 gS/m³·d. The elemental sulfur production was strongly correlated to S/N ratio (r = 0.969, p = 0.03), the highest elemental sulfur production efficiency achieved 92.0% under S/N ratio of 2.0 for treatment of H₂S. Thiobacillus sp. bacteria was the pre-dominated sulfide-dependent denitrifiers (78.2%) before exposing to oxygen, while abundance of Cryseobacterium and unclassified Xanthomonadaceae aerobic sulfide oxidizer dramatically increased up to 40% and 7.3% after aeration. Remarkably increasing production of extracellular polymeric substance (197%) was observed after treatment of CS₂, which might promote the hydrolysis of CS₂ and stabilization of elemental sulfur. This study demonstrated the possibility to apply sulfide-dependent denitrification process for treatment of both hydrophilic and hydrophobic volatile reduced sulfur waste gas with elemental sulfur recovery.

Impact of sulfamethoxazole and organic supplementation on mixotrophic denitrification process: Nitrate removal efficiency and the response of functional microbiota

Recirculating aquaculture systems (RAS) effluent is characterized by low COD to total inorganic nitrogen ratio (C/N), excessive nitrate, and the presence of traces of antibiotics. Hence, it urgently needs to be treated before recycling or discharging. In this study, four denitrification bioreactors at increasing C/N ratios (0, 0.7, 2, and 5) were started up to treat mariculture wastewater under the sulfamethoxazole (SMX) stress, during which the bioreactors performance and the shift of mixotrophic microbial communities were explored. The result showed that during the SMX exposure, organic supplementation enhanced nitrate and thiosulfate removal, and eliminated nitrite accumulation. The denitrification rate was accelerated by increasing C/N from 0 to 2, while it declined at C/N of 5. The decline was ascribed to which SMX reduced the relative abundance of denitrifiers, but improved the capability of dissimilatory nitrogen reduction to ammonia (DNRA) and sulfide production. The direct evidence was the relative abundance of sulfidogenic populations, such as Desulfuromusa, Desulfurocapsa, and Desulfobacter increased under the SMX stress. Moreover, high SMX (1.5 mg L^-1) caused the obvious accumulation of ammonia at C/N of 5 due to the high concentration of sulfide (3.54 ± 1.08 mM) and the enhanced DNRA process. This study concluded that the mixotrophic denitrification process with the C/N of 0.7 presented the best performance in nitrate and sulfur removal and indicated the maximum resistance to SMX.

Nitrous oxide emission mitigation from biological wastewater treatment - A review

Nitrous oxide (N₂O) emitted from wastewater treatment processes has emerged as a focal point for academic and practical research amidst pressing environmental issues. This review presents an updated view on the biological pathways for N₂O production and consumption in addition to the critical process factors affecting N₂O emission. The current research trends including the strain and reactor aspects were then outlined with discussions. Last but not least, the research needs were proposed. The holistic life cycle assessment needs to be performed to evaluate the technical and economic feasibility of the proposed mitigation strategies or recovery options. This review also provides the background information for the proposed future research prospects on N₂O mitigation and recovery technologies. As pointed out, dilution effects of the produced N₂O gas product would hinder its use as renewable energy; instead, its use as an effective oxidizing agent is proposed as a promising recovery option.

Response of simultaneous sulfide and nitrate removal process on acute toxicity of substrate concentration and salinity: Single toxicity and combined toxicity

Simultaneous sulfide and nitrate removal process has performed excellent to treat nitrogen and sulfur pollutants in wastewater treatment. A high salinity stress poses a great challenge to the treatment of highly saline wastewater containing nitrate and sulfide. In addition, sulfide and nitrates are also toxic for the process, and their high concentration would inhibit the process. Therefore, the current work explores the single acute toxic effect and combined toxic effect of salinity and substrate concentration on the performance of the process from the perspective of toxicology. Considering sulfide and nitrate removal performance as an indicator, the IC₅₀ values of sulfide were 293.20 mg S/L and 572.30 mg S/L, respectively; while those of salinity were 6.14% wt (91.78 mS/cm) and 6.63% wt (98.73 mS/cm), respectively. High substrate concentration or salinity resulted in elemental sulfur generation. The molar ratio of generated elemental sulfur to consumed sulfide(R-Sulfate) was close to 1. The response of nitrate reduction product to the elevating substrate concentration was not obvious, while its response to increasing salinity was on the contrary. With the increasing salinity (1.2% wt to 9.6% wt), molar ratio of generated nitrogen gas to consumed nitrate (R-Nitrogen gas) increased from 0.58 to 1, while molar ratio of generated nitrite to consumed nitrate (R-Nitrite) decreased from 0.43 to 0. Factorial analysis test revealed that the combined acute toxicity of substrate and salinity on sulfide oxidization and nitrate reduction were both antagonistic effects.

Tubular Sediment-Water Electrolytic Fuel Cell for Dual-Phase Hexavalent Chromium Reduction

A novel tubular sediment-water electrolytic fuel cell (SWEFC) was fabricated for the reduction of Cr(VI) in a dual-phase system. The approach simulates a standing water body with Cr(VI)-contaminated overlying water (electrolyte) and bottom sediment phase with electrodes placed in both the phases, supplemented with urea as a potential electron donor. Cr(VI) reduction efficiency of 93.2 ± 1.3% from electrolyte (in 1.5 h) and 81.2 ± 1.3% from the sediment phase (in 8 h) with an initial Cr(VI) concentration of 1,000 mg/L was observed in a single-cell configuration. The effect of initial Cr(VI) concentration, variation in sediment salinity and pH, and different electron donors on the SWEFC performance were systematically investigated. SWEFC showed enhanced performance with 2.4-fold higher current (193.9 mA) at 400 mg/L Cr(VI) concentration when cow dung was used as a low-cost alternative to urea as an electron donor. Furthermore, reactor scalability studies were carried out with nine-anode and nine-cathode configuration (3 L electrolyte and 2 kg sediment), and reduction efficiencies of 98.9 ± 0.9% (in 1 h) and 97.6 ± 2.2% (in 8 h) were observed from the electrolyte and sediment phases, respectively. The proposed sediment-water electrolytic fuel cell can be an advanced and environmentally benign strategy for Cr(VI) remediation from contaminated sediment-water interfaces along with electricity generation.

Enhanced microbial nitrate reduction using natural manganese oxide ore as an electron donor

Nitrate contamination of groundwater is a global problem. Enhanced biological nitrate reduction by liquid organics combined with low-cost natural materials (as electron donors) can cost-effectively remove nitrate from groundwater. Dissolved Mn(II) as an electron donor has been thoroughly investigated to support microbial nitrate reduction. However, most Mn in soil and sediments is in solid form, and the ability of solid-phase natural manganese oxide ore (NMO) as electron donor and for supporting microbial nitrate reduction is unknown. Therefore, a microcosm experiment was conducted to bridge this gap in knowledge. The results demonstrated that microbial nitrate reduction (mainly converted to nitrite) was enhanced by NMO (rich in cryptomelane). The electrochemical and X-ray photoelectron spectroscopy analyses suggested that NMO may be oxidized by microbial metabolism. Illumina Miseq sequencing results indicated that Acidovorax spp. played a crucial role in NMO-supported nitrate reduction. Further Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) analyses indicated that bacterial extracellular electron transfer may be one of the mechanisms for the microbial NMO oxidation. The results of our study highlight the potential importance of NMO in nitrate reduction in the natural environment and may pave the way for NMO-assisted technology for nitrate removal from groundwater with less usage of organic electron donors.

Reinforced nitrite supplement by cathode nitrate reduction with a bio-electrochemical system coupled anammox reactor

Anammox has been widely used for the treatment of nitrogen wastewater. However, the problem of stable NO₂^- supplement becomes one of the limiting factors. It is an effective method to obtain NO₂^- by denitrifying the NO₃^-, including the by-product of Anammox. In this study, NO₂^- was reinforced by bio-electrochemical system (BES) through the reaction of partial denitrification in situ in an Anammox reactor. Our results showed that both NO₃^- and NO₂^- can be reduced on the cathode with different Coulombic efficiencies. The reduction of NO₃^- amount increased with an increase in Inf-NO₃^-, which was greater than that of NO₂^-. The conversion amount of NO₃^- was 2.50% ± 17.25% to the theoretical Eff-NO₃^-, and the maximum reduction amount was 23.24% with the highest Coulombic efficiency of 3.56%. High throughput results showed that denitrifying bacteria, such as Limnobacter, Thauera, Denitratisoma, Nitrosomonas and Nitrospira, were attached to the cathode surface and in Anammox granular sludge. This study showed that NO₂^- can be supplied by reducing the by-product NO₃^- with denitrification cathode at Anammox environment in-situ.

Investigating the response of electrogenic metabolism to salinity in saline wastewater treatment for optimal energy output via microbial fuel cells

In the current study, MFCs treating saline wastewater with the different conductivities of 5.0 ± 0.2, 7.7 ± 0.6, 10.5 ± 0.9, 13.0 ± 1.0, 15.3 ± 1.0, and 16.0 ± 0.1 mS/cm were investigated. Increasing salinity drives a considerable shift of microbial communities, and it also affects metabolic pathways in MFCs. Overwhelming acetate oxidizing electron transfer with moderate conductivities between 7.7 and 13.0 mS/cm led to high energy outputs. Power generation at the low conductivities of less than 7.7 mS/cm was restricted by the competition between fermentative bacteria (e.g., Lactobacillus) and exoelectrogens (e.g., Pseudomonas and Shewanella) for substrate utilization. Increasing salinity beyond 13 mS/cm suppressed the fermentation of glucose to butyrate. It also induced sulfidogenesis; sulfide oxidizing bacteria Desulfovibrio (5.2%), Desulfuromonas (3.7%) and exoelectrogen Pseudomonas (1.1%) formed a sulfur-driven current production, thereby resulting in low energy outputs. The present study revealed the effects of ionic conductivity on electrical energy production and provided insights into the dynamics of the MFCs substrate utilization.

Fungal pellets immobilized bacterial bioreactor for efficient nitrate removal at low C/N wastewater

In this study, fungal pellets immobilized denitrifying Pseudomonas stutzeri sp. GF3 was cultivated to establish a bioreactor. The denitrification effect of fixed bacteria with fungal pellets was tested by response surface methodology (RSM). Analysis of the bioreactor showed that the denitrification efficiency reached 100% under the optimal conditions and the denitrification efficiency of the actual wastewater treatment in the stable phase reached 95.91%. Moreover, the organic matter and functional groups in the bioreactor under different C/N conditions were analyzed by fluorescence excitation-emission matrix (EEM) spectra and Fourier transform infrared spectroscopy (FTIR), which revealed that metabolic activities of denitrifying bacteria were enhanced with the increase of C/N. The morphology and structure of bacteria immobilized by fungal pellets explored by scanning electron microscope (SEM) showed the filamentous porous fungal pellets loaded with bacteria. Community structure analysis by high-throughput sequencing demonstrated that strain GF3 might was the dominant strain in bioreactor.