Bioaugmentation of UASB reactors with immobilized Sulfurospirillum barnesii for simultaneous selenate and nitrate removal

A dual-chambered microbial fuel cell with manganese dioxide nano-structured cathode for wastewater treatment

Microbial fuel cells (MFCs) can generate electricity by breaking down organic molecules through sustainable bio-electrochemical processes and wastewater as an energy source. A novel approach to remediate wastewater containing selenite was studied utilizing a selenite-reducing mixed bacterial culture with a nano manganese oxide modified cathode in the MFCs. The modification enhanced electrochemical catalytic activity, extracellular electron transfer rate, chemical oxygen demand (COD) elimination efficiency, and coulombic efficiency. Scanning electron microscopy and energy dispersive x-rays analysis were used to examine a manganese dioxide-coated graphite cathode's surface morphology and chemical composition. The manganese dioxide-coated electrode generated up to 69% higher voltage with 150 ppm selenite concentration than the uncoated graphite electrode. The MFC removed up to 80% of the initial COD of 120 mg l-1and achieved a maximum power density of 1.51 W m-2. The study demonstrates that MFCs can effectively treat selenite-containing wastewater, and modifying the cathode can enhance energy production.

Unraveling mechanisms of selenium recovery by facultative anaerobic bacterium Azospira sp. A9D-23B in distinct reactor configurations

Microbial processes are crucial in the redox transformations of toxic selenium oxyanions. This study focused on isolating an efficient selenate-reducing strain, Azospira sp. A9D-23B, and evaluating its capability to recover extracellular selenium nanoparticles (SeNPs) from selenium-laden wastewater in different reactor setups. Analysis using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) revealed significantly higher extracellular SeNPs production (99%) on the biocathode of the bioelectrochemical (BEC) reactor compared to the conventional bioreactor (65%). Further investigations into the selenate reductase activity of strain A9D-23B revealed distinct mechanisms of selenate reduction in BEC and conventional bioreactor settings. Notably, selenate reductases associated with the outer membrane and periplasm displayed higher activity (18.31 ± 3.8 µmol/mg-min) on the BEC reactor's biocathode compared to the upflow anaerobic conventional bioreactor (3.24 ± 2.9 µmol/mg-min). Conversely, the selenate reductases associated with the inner membrane and cytoplasm exhibited lower activity (5.82 ± 2.2 µmol/mg-min) on the BEC reactor's biocathode compared to the conventional bioreactor (9.18 ± 1.6 µmol/mg-min). However, the comparable kinetic parameter ( K m ) across cellular fractions in both reactors suggest that SeNP localization was influenced by enzyme activity rather than selenate affinity. Overall, the mechanism involved in selenate reduction to SeNPs and the strain's efficiency in detoxifying selenate below levels regulated by the U.S. Environmental Protection Agency has broad implications for sustainable environmental remediation strategies.

Changes in microbial communities during high-rate microbial selenate reduction in an up-flow anaerobic fluidized bed reactor

Biological fluidized bed reactor (FBR) is a promising treatment option for removing selenium oxyanions from wastewater by converting them into elemental selenium. The process can achieve high rates and be efficiently operated at low hydraulic retention times (HRT). However, the effects of HRT on the changes in microbial community in the FBR process have not been previously explored. In this study, dynamic changes of microbial communities both on biofilm carrier and in suspension of a selenate-reducing FBR were explored at various HRTs (0.3-120 h). Based on partial 16S rRNA gene sequencing of the microbial communities, alpha diversity of microbial communities in suspension rather than in the biofilm were impacted by low HRTs (0.3 h-3 h). Members from genera Geobacter, Geoalkalibacter, and Geovibrio were the main selenate-reducing bacteria on carrier throughout the FBR process. Genus Geobacter was dominant in FBR carrier at HRT of 24 h-120 h, whereas Geoalkalibacter and Geovibrio dominated at low HRT of 0.3 h-6 h. Suspended microbial communities detected in the FBR effluent were more sensitive to HRT changes than that in biofilm. "Shock loading" at HRT of 0.3 h had a great impact on microbial community compositions both in the biofilm and effluent. Reactor operation in batch mode and long HRT of 24 h helped recover the community from "shock loading" and improved selenite reduction and ethanol oxidation. Redundancy analysis revealed that HRT, influent pH and selenate loading were key operational parameters impacting both the FBR performance and the composition of microbial communities associated with both the FBR carrier and effluent. Overall, the microbial communities in FBR biofilm flexibly responded to the changes of HRT and showed resilience to the temporary shock loading, enabling efficient selenate removal.

A Bibliometric Analysis of Research on Selenium in Drinking Water during the 1990-2021 Period: Treatment Options for Selenium Removal

A bibliometric analysis based on the Scopus database was carried out to summarize the global research related to selenium in drinking water from 1990 to 2021 and identify the quantitative characteristics of the research in this period. The results from the analysis revealed that the number of accumulated publications followed a quadratic growth, which confirmed the relevance this research topic is gaining during the last years. High research efforts have been invested to define safe selenium content in drinking water, since the insufficient or excessive intake of selenium and the corresponding effects on human health are only separated by a narrow margin. Some important research features of the four main technologies most frequently used to remove selenium from drinking water (coagulation, flocculation and precipitation followed by filtration; adsorption and ion exchange; membrane-based processes and biological treatments) were compiled in this work. Although the search of technological options to remove selenium from drinking water is less intensive than the search of solutions to reduce and eliminate the presence of other pollutants, adsorption was the alternative that has received the most attention according to the research trends during the studied period, followed by membrane technologies, while biological methods require further research efforts to promote their implementation.

Sequential removal of selenate, nitrate and sulfate and recovery of elemental selenium in a multi-stage bioreactor process with redox potential feedback control

Bioreduction can facilitate oxyanions removal from wastewater. However, simultaneously removing selenate, nitrate and sulfate and recovering high-purity elemental selenium (Se⁰) from wastewater by a single system is difficult and may lead to carcinogenic selenium monosulfide (SeS) formation. To solve this issue, a two-stage biological fluidized bed (FBR) process with ethanol dosing based on oxidation-reduction potential (ORP) feedback control was developed in this study. FBR1 performance was first evaluated at various ORP setpoints (between -520 and -360 mV vs. Ag/AgCl) and elevated sulfate concentration. Subsequently, ethanol-fed FBR2 was used to reduce sulfate from FBR1 effluent, followed by an aerated sulfide oxidation reactor (SOR). At - 520 mV≤ ORPs≤ -480 mV, FBR1 removed 100 ± 0.1% nitrate and 99.7 ± 0.3% selenate without sulfate reduction. At ORPs ≥ -440 mV, selenate reduction was incomplete, whereas nitrate removal remained stable. Se⁰ recovery efficiency from FBR1 effluent was 37.5% with 71% Se purity. FBR2 converted 86% of the remaining sulfate in FBR1 effluent to hydrogen sulfide, but the over-oxidation of dissolved sulfide in SOR decreased the overall sulfate removal efficiency to ~46.3%. Overall, the two-stage FBR process with ORP feedback dosing of ethanol was effective for sequentially removing selenate, nitrate and sulfate and recovering Se⁰ from wastewater.

Microbial selenate detoxification linked to elemental sulfur oxidation: Independent and synergic pathways

Elevated selenium levels in the environment, with soluble selenate [Se(VI)] as the common chemical species, pose a severe threat to human health. Anaerobic Se(VI) bioreduction is a promising approach for selenium detoxification, and various organic/inorganic electron donors have proved effective in supporting this bioprocess. Nevertheless, autotrophic Se(VI) bioreduction driven by solid inorganic electron donors is still not fully understood. This work is the first to employ elemental sulfur [S(0)] as electron donor to support Se(VI) bioreduction. A batch trial with mixed culture demonstrated the feasibility of this bioprocess, with Se(VI) removal efficiency of 92.4 ± 0.7% at an initial Se(VI) concentration of 10 mg/L within 36 h. Continuous column tests showed that increased initial concentration, flow rate, and introduction of NO₃^--N depressed Se(VI) removal. Se(VI) was mainly bioreduced to solid elemental Se with trace selenite in the effluent, while S(0) was oxidized to SO₄^2-. Enrichment of Thiobacillus, Desulfurivibrio, and Sulfuricurvum combined with upregulation of genes serA, tatC, and soxB indicated Se(VI) bioreduction was coupled to S(0) oxidation. Thiobacillus performed S(0) oxidation and Se(VI) reduction independently. Intermediate metabolites as volatile fatty acids, hydrogen and methane from S(0) oxidation were utilized by heterotrophic Se(VI) reducers for Se(VI) detoxification, indicative of microbial synergy.

A critical review on sulfur reduction of aqueous selenite: Mechanisms and applications

Selenite, which is extremely toxic at high concentrations, can easily be enriched in natural aquatic environments due to human activities, which causes great harm to ecosystems. Sulfur reduction can effectively reduce soluble selenite in large quantities to nontoxic solid elemental selenium, which plays a significant role in controlling the toxicity and cycle of selenium. In view of the bright prospects of the sulfur reduction reaction of selenite, this review comprehensively summarizes the continuous development in the sulfidation of selenite. First, the geochemical characteristics of aqueous selenium in different sulfur systems involving species distribution and various phase types at Eh-pH conditions were summarized. Second, sulfur reductions of selenite with chemical sulfide in natural water environments, sulfur reductase and extracellular polymer substances containing thiol groups in sulfate-reducing bacteria have been reviewed to further understand the corresponding mechanisms, rates and influencing factors. Furthermore, applications of sulfur reduction of selenium, including removal of selenium, enrichment of selenium, synthesis of selenoproteins and prevention of leakage of selenium, were also summarized. Finally, this review identified future research needs for the sulfidation of selenite for environmental applications.

Magnetic properties of biogenic selenium nanomaterials

Bioreduction of selenium oxyanions to elemental selenium is ubiquitous; elucidating the properties of this biogenic elemental selenium (BioSe) is thus important to understand its environmental fate. In this study, the magnetic properties of biogenic elemental selenium nanospheres (BioSe-Nanospheres) and nanorods (BioSe-Nanorods) obtained via the reduction of selenium(IV) using anaerobic granular sludge taken from an upflow anaerobic sludge blanket (UASB) reactor treating paper and pulp wastewater were investigated. The study indicated that the BioSe nanomaterials have a strong paramagnetic contribution with some ferromagnetic component due to the incorporation of Fe(III) (high-spin and low-spin species) as indicated by electron paramagnetic resonance (EPR). The paramagnetism did not saturate up to 50,000 Oe at 5 K, and the hysteresis curve showed the coercivity of 100 Oe and magnetic moment saturation around 10 emu. X-ray photoelectron spectroscopy (XPS) and EPR evidenced the presence of Fe(III) in the nanomaterial. Signals for Fe(II) were observed neither in EPR nor in XPS ruling out its presence in the BioSe nanoparticles. Fe(III) being abundantly present in the sludge likely got entrapped in the extracellular polymeric substances (EPS) coating the biogenic nanomaterials. The presence of Fe(III) in BioSe nanomaterial increases the mobility of Fe(III) and may have an effect on phytoplankton growth in the environment. Furthermore, as supported by the literature, there is a potential to exploit the magnetic properties of BioSe nanomaterials in drug delivery systems as well as in space refrigeration.

Addition of Trichocladium canadense to an anaerobic membrane bioreactor: evaluation of the microbial composition and reactor performance

Membrane bioreactors are powerful systems for wastewater treatment and the removal of toxic compounds. However, membrane biofouling stands in the way of their widespread usage. In this study, the saprophytic fungus Trichocladium canadense was used as the bioaugmentor in an anaerobic membrane bioreactor (AnMBR) and its impact on membrane biofouling, biogas production, the microbial communities of the reactor and removal of the common antibiotics erythromycin (ERY), sulfamethoxazole (SMX) and tetracycline (TET) from synthetic wastewater was investigated. The results indicated that through bioaugmentation with 20% T. canadense, membrane biofouling was slowed by 25%, the chemical oxygen demand removal increased by 16% and a higher efficiency removal of ERY and SMX was achieved. The presence of T. canadense significantly increased the abundance and diversity of the biofilm archaeal community and the bacterial phylum Firmicutes, a known bio-foulant.

Selenium respiration in anaerobic bacteria: Does energy generation pay off?

Selenium (Se) respiration in bacteria was revealed for the first time at the end of 1980s. Although thermodynamically-favorable, energy-dense and documented in phylogenetically-diverse bacteria, this metabolic process appears to be accompanied by a number of challenges and numerous unanswered questions. Selenium oxyanions, SeO₄^2- and SeO₃^2-, are reduced to elemental Se (Se⁰) through anaerobic respiration, the end product being solid and displaying a considerable size (up to 500 nm) at the bacterial scale. Compared to other electron acceptors used in anaerobic respiration (e.g. N, S, Fe, Mn, and As), Se is one of the few elements whose end product is solid. Furthermore, unlike other known bacterial intracellular accumulations such as volutin (inorganic polyphosphate), S⁰, glycogen or magnetite, Se⁰ has not been shown to play a nutritional or ecological role for its host. In the context of anaerobic respiration of Se oxyanions, biogenic Se⁰ appears to be a by-product, a waste that needs proper handling, and this raises the question of the evolutionary implications of this process. Why would bacteria use a respiratory substrate that is useful, in the first place, and then highly detrimental? Interestingly, in certain artificial ecosystems (e.g. upflow bioreactors) Se⁰ might help bacterial cells to increase their density and buoyancy and thus avoid biomass wash-out, ensuring survival. This review article provides an in-depth analysis of selenium respiration (model selenium respiring bacteria, thermodynamics, respiratory enzymes, and genetic determinants), complemented by an extensive discussion about the evolutionary implications and the properties of biogenic Se⁰ using published and original/unpublished results.

Hydrodynamics- and hydrochemistry-affected microbial selenate reduction in aquifer: Performance and mechanisms

Selenate [Se(VI)] with higher content in groundwater will be harmful for human beings. Hence, effective treatment for Se(VI) in aquifer should be conducted reasonably. Microbial reduction of Se(VI) to elemental selenium with weak movability and toxicity has attracted significant attention due to its high efficiency and no secondary contamination. However, hydrodynamic and hydrochemical influences with corresponding mechanisms during Se(VI) bioreduction are still not clear. In this study, influences of flow rate, initial Se(VI) and organic concentrations, coexisting nitrate were evaluated. Se(VI) removal efficiency and capacity reached 96.42 ± 6.82% and 41.28 ± 3.41 (g/m³·d) with flow rate of 0.56 mL/min, initial Se(VI) and chemical organic demand concentrations of 10 mg/L and 400 mg/L. Dechloromonas and Pseudomonas were presumably contributed to Se(VI) reduction, with upregulated serA and tatC genes. Solid Se⁰ was identified as the final product from Se(VI) reduction. These results will be beneficial for the further comprehending of Se(VI) remediation in aquifer.

High-rate biological selenate reduction in a sequencing batch reactor for recovery of hexagonal selenium

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L^-1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L^-1 day^-1) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L^-1 day^-1, which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe^-1day^-1g_biomass^-1, which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H₂Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.

Optimization of nitrate and selenate reduction in an ethanol-fed fluidized bed reactor via redox potential feedback control

Electron donors are a major cost-factor in biological removal of oxyanions, such as nitrate and selenate from wastewater. In this study, an online ethanol dosing strategy based on feedback control of oxidation-reduction potential (ORP) was designed to optimize the performance of a lab-scale fluidized bed reactor (FBR) in treating selenate and nitrate (5 mM each) containing wastewater. The FBR performance was evaluated at various ORP setpoints ranging between -520 mV and -240 mV (vs. Ag/AgCl). Results suggested that both nitrate and selenate were completely removed at ORPs between -520 mV and -360 mV, with methylseleninic acid, selenocyanate, selenosulfate and ammonia being produced at low ORPs between -520 mV and -480 mV, likely due to overdosing of ethanol. At ORPs between -300 mV and -240 mV, limited ethanol dosing resulted in an apparent decline in selenate removal whereas nitrate removal remained stable. Resuming the ORP to -520 mV successfully restored complete selenate reduction. An optimal ORP of -400 mV was identified for the FBR, whereby selenate and nitrate were nearly completely removed with a minimal ethanol consumption. Overall, controlling ORP via feedback-dosing of the electron donor was an effective strategy to optimize FBR performance for reducing selenate and nitrate in wastewater.

High-rate microbial selenate reduction in an up-flow anaerobic fluidized bed reactor (FBR)

Wastewater contaminated with high concentrations of selenium oxyanions requires treatment prior to discharge. Biological fluidized bed reactors (FBRs) can be an option for removing selenium oxyanions from wastewater by converting them into elemental selenium, which can be separated from the treated effluent. In this study, a lab-scale FBR was constructed with granular activated carbon as biofilm carrier and inoculated with a consortium of selenate reducing bacteria enriched from environmental samples. The FBR was loaded with an influent containing ethanol (10 mM) and selenate (10 mM) as the microbial electron donor and acceptor, respectively. The performance of the FBR in reducing selenate was evaluated under various hydraulic retention times (HRTs) (120 h, 72 h, 48 h, 24 h, 12 h, 6 h, 3 h, 1 h and 20 min). After process acclimatization, selenate was completely removed with no notable selenite produced when the HRT was stepwise decreased from 120 h to 6 h. However, decreasing the HRT to 3 h resulted in selenite accumulation (0.17 ± 0.023 mM) in the effluent although selenate removal efficiency remained at 99.8 ± 0.20%. At 1 h HRT, the FBR removed 90.8 ± 1.4% of the selenate at a rate of 9.6 ± 0.15 mM h^-1, which is the highest selenate reduction rate reported in the literature so far. However, 1 h HRT resulted in notable selenite accumulation (up to 2.4 ± 0.27 mM). Further decreasing the HRT to 20 min resulted in a notable decline in selenate reduction. Selenate reduction recovered from the "shock loading" after the HRT was increased back to 3 h. However, selenite still accumulated until the FBR was operated in batch mode for 6 days. This study affirmed that FBR is a promising treatment option for selenate-rich wastewater, and the process can be efficiently operated at low HRTs.

Microbial selenate reduction in membrane biofilm reactors using ethane and propane as electron donors

Selenate (Se(VI)) contamination in groundwater is one of major concerns for human health, in particular in shale gas extraction sites. Microbial selenate reduction coupled to methane (CH₄) oxidation has been demonstrated very recently. Little is known whether ethane (C₂H₆) and butane (C₃H₈) are able to drive selenate reduction, although they are also important components in shale gas. In this study, we demonstrated Se(VI) bio-reduction could be achieved using C₂H₆ and C₃H₈ as electron donors and carbon sources. Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX) confirmed elemental Se (Se⁰) was the major final product formed from Se(VI) bio-reduction. Polyhydroxyalkanoates (PHAs) were generated in the biofilms as the internal electron-storage materials, which were consumed for sustaining Se(VI) bio-reduction in absence of C₂H₆ and C₃H₈. Microbial community analysis showed that two genera capable of oxidizing gaseous alkanes dominated in the biofilms, including Mycobacterium (in both C₂H₆ and C₃H₈-fed biofilms) and Rhodococcus (in C₃H₈-fed biofilm). In addition, several potential Se(VI) reducers (e.g., Variovorax) were detected in the biofilms. Investigation of Communities by Reconstruction of Unobserved States analysis supported that predictive genes associated with alkanes oxidation, denitrification and PHAs cycle were enriched in the biofilms. These findings offer insights into the process of selenate reduction driven by C₂H₆ and C₃H₈, which ultimately may help to develop a solution to use shale gas for groundwater remediation, especially near shale gas exploitation sites.

Effect of Nitrate and Perchlorate on Selenate Reduction in a Sequencing Batch Reactor

Selenate removal from a water body is being vigorously debated owing to severe health impact, but inhibitions of coexisting anions have been reported. To suggest a viable treatment option, this study investigates the effect of nitrate and perchlorate on selenate reduction in a laboratory-scale sequencing batch reactor. The experimental design tests how competing electron acceptors (NO3− and ClO4−) and electron donor (acetate) limitations affect selenate reduction in the reactor. Results show that the reactor achieves almost complete selenate reduction within the initial concentration ranges of 0.1–1 mM by enriching selenate-reducing bacteria with appropriate temperature (30 °C) and acclimation period (50 days). We monitored simultaneous selenate and nitrate reduction in the reactor without specific inhibition due to a difference in microbial growth strategy related to electron donor status. Lack of perchlorate-reducing bacteria makes perchlorate addition (0.2 mM) not to be closely associated with dissimilative perchlorate reduction. These results provide information that can help us to understand the effect of competing electron acceptors on selenate reduction and the kinetics of potential parallel reactions in the reactor.

Understanding microbial ecology can help improve biogas production in AD

454-Pyrosequencing and lipid fingerprinting were used to link anaerobic digestion (AD) process parameters (pH, alkalinity, volatile fatty acids (VFAs), biogas production and methane content) with the reactor microbial community structure and composition. AD microbial communities underwent stress conditions after changes in organic loading rate and digestion substrates. 454-Pyrosequencing analysis showed that, irrespectively of the substrate digested, methane content and pH were always significantly, and positively, correlated with community evenness. In AD, microbial communities with more even distributions of diversity are able to use parallel metabolic pathways and have greater functional stability; hence, they are capable of adapting and responding to disturbances. In all reactors, a decrease in methane content to <30% was always correlated with a 50% increase of Firmicutes sequences (particularly in operational taxonomic units (OTUs) related to Ruminococcaceae and Veillonellaceae). Whereas digesters producing higher methane content (above 60%), contained a high number of sequences related to Synergistetes and unidentified bacterial OTUs. Finally, lipid fingerprinting demonstrated that, under stress, the decrease in archaeal biomass was higher than the bacterial one, and that archaeal Phospholipid etherlipids (PLEL) levels were correlated to reactor performances. These results demonstrate that, across a number of parameters such as lipids, alpha and beta diversity, and OTUs, knowledge of the microbial community structure can be used to predict, monitor, or optimise AD performance.

Biological treatment of selenium-laden wastewater containing nitrate and sulfate in an upflow anaerobic sludge bed reactor at pH 5.0

This study investigated the removal of selenate (SeO₄^2-), sulfate (SO₄^2-) and nitrate (NO₃^-) at different influent pH values ranging from 7.0 to 5.0 and 20 °C in an upflow anaerobic sludge blanket (UASB) reactor using lactate as an electron donor. At pH 5.0, the UASB reactor showed a 20-30% decrease in reactor performance compared to operation at pH 5.5 to 7.0, reaching removal efficiencies of 79%, 15%, 43% and 61% for NO₃^-, SO₄^2-, Se_total and Se_diss, respectively. However, the reactor stability was an issue upon lowering the pH to 5.0 and further experiments are recommended. The sludge formed during low pH operation had a fluffy, floc-like appearance with filamentous structure, possibly due to the low polysaccharide (PS) to protein (PN) ratio (0.01 PS/PN) in the soluble extracellular polymeric substances (EPS) matrix of the biomass. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) analysis of the sludge confirmed Se oxyanion reduction and deposition of Se⁰ particles inside the biomass. Microbial community analysis using Illumina MiSeq sequencing revealed that the families of Campylobacteraceae and Desulfomicrobiaceae were the dominant phylotypes throughout the reactor operation at approximately 23% and 10% relative abundance, respectively. Furthermore, approximately 10% relative abundance of both Geobacteraceae and Spirochaetaceae was observed in the granular sludge during the pH 5.0 operation. Overall, this study demonstrated the feasibility of UASB operation at pH values ranging from 7.0 to 5.0 for removing Se and other oxyanions from wastewaters.

Effects of sulfate on simultaneous nitrate and selenate removal in a hydrogen-based membrane biofilm reactor for groundwater treatment: Performance and biofilm microbial ecology

Effects of sulfate on simultaneous nitrate and selenate removal in a hydrogen-based membrane biofilm reactor (MBfR) for groundwater treatment was identified with performance and biofilm microbial ecology. In whole operation, MBfR had almost 100% removal of nitration even with 50 mg mL^-1 sulfate. Moreover, selenate degradation increased from 95% to approximate 100% with sulfate addition, indicating that sulfate had no obvious effects on nitrate degradation, and even partly promoted selenate removal. Short-term sulfate effect experiment further showed that Gibbs free energy of reduction (majority) and abiotic sulfide oxidation (especially between sulfate and selenate) contributed to degradable performance with sulfate. Microbial ecology showed that high percentage of Hydrogenophaga (≥75%) was one of the contributors for the stable and efficient nitrate degradation. Chemoheterotrophy (ratio>0.3) and dark hydrogen oxidation (ratio>0.3) were the majority of functional profile for biofilm in MBfR, and sulfate led to profiles of sulfate respiration and respiration of sulfur compounds in biofilm. Additionally, no special bacteria for selenate degradation was identified in biofilm microbial ecology, and selenate degradation was relied on Hydrogenophaga (75% of ecology percentage with sulfate addition) and Desulfovibrionaceae (4% of ecology percentage with sulfate addition). But with overloading sulfate, Desulfovibrionaceae was prior to sulfate degradation for energy supply and thus inhibited selenate removal.

The effect of electron competition on chromate reduction using methane as electron donor

We studied the effect of electron competition on chromate (Cr(VI)) reduction in a methane (CH₄)-based membrane biofilm reactor (MBfR), since the reduction rate was usually limited by electron supply. A low surface loading of SO₄^2- promoted Cr(VI) reduction. The Cr(VI) removal percentage increased from 60 to 70% when the SO₄^2- loading increased from 0 to 4.7 mg SO₄^2-/m²-d. After the SO₄^2- loading decreased back to zero, the Cr(VI) removal further increased to 90%, suggesting that some sulfate-reducing bacteria (SRB) stayed in the reactor to reduce Cr(VI). However, a high surface loading of SO₄^2- (26.6 mg SO₄^2-/m²-d) significantly slowed down the Cr(VI) reduction to 40% removal, which was probably due to competition between Cr(VI) and SO₄^2- reduction. Similarly, when 0.5 mg/L of Se(VI) was introduced into the MBfR, Cr(VI) removal percentage slightly decreased to 60% and then increased to 80% when input Se(VI) was removed again. The microbial community strongly depended on the loadings of Cr(VI) and SO₄^2-. In the sulfate effect experiment, three genera were dominant. Based on the correlation between the abundances of the three genera and the loadings of Cr(VI) and SO₄^2-, we conclude that Methylocystis, a type II methanotroph, reduced both Cr(VI) and sulfate, Meiothermus only reduced Cr(VI), and Ferruginibacter only reduced SO₄^2-.

Bioremediation of Wastewater by Iron Oxide-Biochar Nanocomposites Loaded with Photosynthetic Bacteria

It has been reported that bacteria-mediated degradation of contaminants is a practical and innocuous wastewater treatment. In addition, iron oxide nanoparticles (NP) are wastewater remediation agents with great potentials due to their strong adsorption capacity, chemical inertness and superparamagnetism. Therefore, a combination of NPs and microbes could produce a very desirable alternative to conventional wastewater treatment. For this purpose, we first prepared Fe₃O₄/biochar nano-composites, followed by loading photosynthetic bacteria (PSB) onto them. It was found that Fe₃O₄/biochar nano-composites exhibited a high adsorption capacity for PSB (5.45 × 10⁹ cells/g). The efficiency of wastewater pollutants removal by this PSB/Fe₃O₄/biochar agent was then analyzed. Our results indicated that when loaded onto Fe₃O₄/biochar nano-composites, PSB’s nutrient removal capability was significantly enhanced (P < 0.05). This agent removed 83.1% of chemical oxygen demand, 87.5% of NH₄⁺, and 92.1% of PO₄^3- from the wastewater in our study. Our experiments also demonstrated that such composites are outstanding recyclable agents. Their nutrient removal capability remained effective even after five cycles. In conclusion, we found the PSB/Fe₃O₄/biochar composites as a very promising material for bioremediation in the wastewater treatment.

Selenium: environmental significance, pollution, and biological treatment technologies

Selenium is an essential trace element needed for all living organisms. Despite its essentiality, selenium is a potential toxic element to natural ecosystems due to its bioaccumulation potential. Though selenium is found naturally in the earth's crust, especially in carbonate rocks and volcanic and sedimentary soils, about 40% of the selenium emissions to atmospheric and aquatic environments are caused by various industrial activities such as mining-related operations. In recent years, advances in water quality and pollution monitoring have shown that selenium is a contaminant of potential environmental concern. This has practical implications on industry to achieve the stringent selenium regulatory discharge limit of 5μgSeL(-1) for selenium containing wastewaters set by the United States Environmental Protection Agency. Over the last few decades, various technologies have been developed for the treatment of selenium-containing wastewaters. Biological selenium reduction has emerged as the leading technology for removing selenium from wastewaters since it offers a cheaper alternative compared to physico-chemical treatments and is suitable for treating dilute and variable selenium-laden wastewaters. Moreover, biological treatment has the advantage of forming elemental selenium nanospheres which exhibit unique optical and spectral properties for various industrial applications, i.e. medical, electrical, and manufacturing processes. However, despite the advances in biotechnology employing selenium reduction, there are still several challenges, particularly in achieving stringent discharge limits, the long-term stability of biogenic selenium and predicting the fate of bioreduced selenium in the environment. This review highlights the significance of selenium in the environment, health, and industry and biotechnological advances made in the treatment of selenium contaminated wastewaters. The challenges and future perspectives are overviewed considering recent biotechnological advances in the management of these selenium-laden wastewaters.

Effect of temperature on selenium removal from wastewater by UASB reactors

The effect of temperature on selenium (Se) removal by upflow anaerobic sludge blanket (UASB) reactors treating selenate and nitrate containing wastewater was investigated by comparing the performance of a thermophilic (55 °C) versus a mesophilic (30 °C) UASB reactor. When only selenate (50 μM) was fed to the UASB reactors (pH 7.3; hydraulic retention time 8 h) with excess electron donor (lactate at 1.38 mM corresponding to an organic loading rate of 0.5 g COD L(-1) d(-1)), the thermophilic UASB reactor achieved a higher total Se removal efficiency (94.4 ± 2.4%) than the mesophilic UASB reactor (82.0 ± 3.8%). When 5000 μM nitrate was further added to the influent, total Se removal was again better under thermophilic (70.1 ± 6.6%) when compared to mesophilic (43.6 ± 8.8%) conditions. The higher total effluent Se concentration in the mesophilic UASB reactor was due to the higher concentrations of biogenic elemental Se nanoparticles (BioSeNPs). The shape of the BioSeNPs observed in both UASB reactors was different: nanospheres and nanorods, respectively, in the mesophilic and thermophilic UASB reactors. Microbial community analysis showed the presence of selenate respirers as well as denitrifying microorganisms.

Relating Anaerobic Digestion Microbial Community and Process Function

Anaerobic digestion (AD) involves a consortium of microorganisms that convert substrates into biogas containing methane for renewable energy. The technology has suffered from the perception of being periodically unstable due to limited understanding of the relationship between microbial community structure and function. The emphasis of this review is to describe microbial communities in digesters and quantitative and qualitative relationships between community structure and digester function. Progress has been made in the past few decades to identify key microorganisms influencing AD. Yet, more work is required to realize robust, quantitative relationships between microbial community structure and functions such as methane production rate and resilience after perturbations. Other promising areas of research for improved AD may include methods to increase/control (1) hydrolysis rate, (2) direct interspecies electron transfer to methanogens, (3) community structure-function relationships of methanogens, (4) methanogenesis via acetate oxidation, and (5) bioaugmentation to study community-activity relationships or improve engineered bioprocesses.

Bioaugmentation and its application in wastewater treatment: A review

Bioaugmentation (the process of adding selected strains/mixed cultures to wastewater reactors to improve the catabolism of specific compounds, e.g. refractory organics, or overall COD) is a promising technique to solve practical problems in wastewater treatment plants, and enhance removal efficiency. The potential of this option can now be enhanced in order to take advantage of important advances in the fields of microbial ecology, molecular biology, immobilization techniques and advanced bioreactor design. Reports on bioaugmentation in WWT show the difficulties in evaluating the potential parameters involved, leading frequently to inconclusive outcomes. Many studies have been carried out on the basis of trial-and-error approaches, and it has been reported that reactors bioaugmented with pure cultures often fail to perform as well as the pure cultures under laboratory conditions. As an interesting technical challenge, the feasibility of bioaugmentation should ultimately be assessed by data from field implementation, and this review highlights several promising areas to explore in the future.

Selenium biomineralization for biotechnological applications

Selenium (Se) is not only a strategic element in high-tech electronics and an essential trace element in living organisms, but also a potential toxin with low threshold concentrations. Environmental biotechnological applications using bacterial biomineralization have the potential not only to remove selenium from contaminated waters, but also to sequester it in a reusable form. Selenium biomineralization has been observed in phylogenetically diverse microorganisms isolated from pristine and contaminated environments, yet it is one of the most poorly understood biogeochemical processes. Microbial respiration of selenium is unique because the microbial cells are presented with both soluble (SeO(4)(2-) and SeO(3)(2-)) and insoluble (Se(0)) forms of selenium as terminal electron acceptor. Here, we highlight selenium biomineralization and the potential biotechnological uses for it in bioremediation and wastewater treatment.

Ecology and biotechnology of selenium-respiring bacteria

In nature, selenium is actively cycled between oxic and anoxic habitats, and this cycle plays an important role in carbon and nitrogen mineralization through bacterial anaerobic respiration. Selenium-respiring bacteria (SeRB) are found in geographically diverse, pristine or contaminated environments and play a pivotal role in the selenium cycle. Unlike its structural analogues oxygen and sulfur, the chalcogen selenium and its microbial cycling have received much less attention by the scientific community. This review focuses on microorganisms that use selenate and selenite as terminal electron acceptors, in parallel to the well-studied sulfate-reducing bacteria. It overviews the significant advancements made in recent years on the role of SeRB in the biological selenium cycle and their ecological role, phylogenetic characterization, and metabolism, as well as selenium biomineralization mechanisms and environmental biotechnological applications.

Ammonia oxidizers in a pilot-scale multilayer rapid infiltration system for domestic wastewater treatment

A pilot-scale multilayer rapid infiltration system (MRIS) for domestic wastewater treatment was established and efficient removal of ammonia and chemical oxygen demand (COD) was achieved in this study. The microbial community composition and abundance of ammonia oxidizers were investigated. Efficient biofilms of ammonia oxidizers in the stationary phase (packing material) was formed successfully in the MRIS without special inoculation. DGGE and phylogenetic analyses revealed that proteobacteria dominated in the MRIS. Relative abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) showed contrary tendency. In the flowing phase (water effluent), AOA diversity was significantly correlated with the concentration of dissolve oxygen (DO), NO₃-N and NH₃-N. AOB abundance was significantly correlated with the concentration of DO and chemical oxygen demand (COD). NH₃-N and COD were identified as the key factors to shape AOB community structure, while no variable significantly correlated with that of AOA. AOA might play an important role in the MRIS. This study could reveal key environmental factors affecting the community composition and abundance of ammonia oxidizers in the MRIS.

Nitrate shaped the selenate-reducing microbial community in a hydrogen-based biofilm reactor

To study the effect of nitrate (NO3(-)) on selenate (SeO4(2-)) reduction, we tested a H2-based biofilm with a range of influent NO3(-) loadings. When SeO4(2-) was the only electron acceptor (stage 1), 40% of the influent SeO4(2-) was reduced to insoluble elemental selenium (Se(0)). SeO4(2-) reduction was dramatically inhibited when NO3(-) was added at a surface loading larger than 1.14 g of N m(-2) day(-1), when H2 delivery became limiting and only 80% of the input NO3(-) was reduced (stage 2). In stage 3, when NO3(-) was again removed from the influent, SeO4(2-) reduction was re-established and increased to 60% conversion to Se(0). SeO4(2-) reduction remained stable at 60% in stages 4 and 5, when the NO3(-) surface loading was re-introduced at ≤ 0.53 g of N m(-2) day(-1), allowing for complete NO3(-) reduction. The selenate-reducing microbial community was significantly reshaped by the high NO3(-) surface loading in stage 2, and it remained stable through stages 3-5. In particular, the abundance of α-Proteobacteria decreased from 30% in stage 1 to less than 10% of total bacteria in stage 2. β-Proteobacteria, which represented about 55% of total bacteria in the biofilm in stage 1, increased to more than 90% of phylotypes in stage 2. Hydrogenophaga, an autotrophic denitrifier, was positively correlated with NO3(-) flux. Thus, introducing a NO3(-) loading high enough to cause H2 limitation and suppress SeO4(2-) reduction had a long-lasting effect on the microbial community structure, which was confirmed by principal coordinate analysis, although SeO4(2-) reduction remained intact.

Environmental selenium research: from microscopic processes to global understanding

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Methanogen community structure-activity relationship and bioaugmentation of overloaded anaerobic digesters

Accumulation of acids in anaerobic digesters after organic overload can inhibit or stop CH4 production. Therefore, methods to reduce acid concentrations would be helpful. One potential method to improve recovery involves bioaugmentation, addition of specific microorganisms to improve performance. In this study, transiently overloaded digesters were bioaugmented with a propionate-degrading enrichment culture in an effort to decrease recovery time. Biomass samples from 14 different, full-scale anaerobic digesters were screened for specific methanogenic activity (SMA) against propionate; the microbial communities were also compared. SMA values spanned two orders of magnitude. Principal component analysis of denaturing gradient gel electrophoresis (DGGE) banding patterns for a functional gene (mcrA) suggested an underlying community structure-activity relationship; the presence of hydrogenotrophic methanogens closely related to Methanospirillum hungatei and Methanobacterium beijingense was associated with high propionate SMA values. The biomass sample demonstrating the highest SMA was enriched for propionate degrading activity and then used to bioaugment overloaded digesters. Bioaugmented digesters recovered more rapidly following the organic overload, requiring approximately 25 days (2.5 solids retention times (SRTs)) less to recover compared to non-bioaugmented digesters. Benefits of bioaugmentation continued for more than 12 SRTs after organic overload. Bioaugmentation is a promising approach to decrease recovery time after organic overload.

Methane production from oleate: assessing the bioaugmentation potential of Syntrophomonas zehnderi

The potential for improving long-chain fatty acids (LCFA) conversion to methane was evaluated by bioaugmenting a non-acclimated anaerobic granular sludge with Syntrophomonas zehnderi. Batch bioaugmentation assays were performed with and without the solid microcarrier sepiolite, using 1 mM oleate as sole carbon and energy source. When S. zehnderi was added to the anaerobic sludge methane production from oleate was faster. High methane yields, i.e. 89 ± 5% and 72 ± 1%, were observed in bioaugmented assays in the absence and presence of sepiolite, respectively. Sepiolite stimulated a faster methane production from oleate and prevented the accumulation of acetate. Acetoclastic activity was affected by oleate in the absence of sepiolite, where methane production rate was 26% lower than in assays with microcarrier.