Influence of pH, EDTA/Fe(II) ratio, and microbial culture on Fe(II)-mediated autotrophic denitrification

Long-term effect of light rare earth element neodymium on Anammox process

During the mining of rare earth minerals, the application of neodymium-containing manures, and the treatment of spent neodymium iron boron magnet, the generation of ammonia wastewater containing neodymium is increasing. Thus, the effects of neodymium (Nd(III)) on anaerobic ammonium oxidation (Anammox) were investigated from the aspects of performance, kinetics, statistics, microbial community and sludge morphology, and the recovery strategy of EDTA-2Na wash was discussed. The nitrogen removal efficiency of the Anammox reactor decreased significantly and eventually collapsed at the Nd(III) dosing levels of 20 and 40 mg L-1, respectively. And the toxicity of Nd(III) to AnAOB was determined by the amount internalized into the cells. The EDTA-2Na wash successfully increased the total nitrogen removal rate (TNRR) of Nd(III)-inhibited Anammox to 41.60% of its initial value within 30 days, and the modified Boltzmann model accurately simulated this recovery process. The transient and extended effects of Nd(III), self-recovery, and EDTA-2Na wash on Anammox were effectively assessed using a one-sample t-test. 16S rRNA gene sequencing indicated that Nd(III) remarkably decreased the relative abundance of Planctomycetes and Candidatus Brocadia. The scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) revealed crystal-like neodymium particles on the surface of Anammox sludge. The above-mentioned results demonstrate that the concentration of Nd(III) should be below the toxicity threshold (20 mg L-1) when treating ammonia wastewater containing neodymium by Anammox, and also emphasize the importance of an appropriate recovery strategy.

A critical review of inverse fluidized bed reactors-start-up optimization strategies and wastewater treatment

A critical evaluation of strategies used for reducing start-up time and biological wastewater treatment using an inverse fluidized bed reactor (IFBR) was done. The start-up of an IFBR is one of the most important, time-consuming, and limiting steps in wastewater treatment using biofilm reactors. Evaluation of different strategies used by various researchers is helpful in future research works with this reactor. Different types of treated wastewater, the effect of wastewater characteristics, carriers used, and reactor hydrodynamics on the reactor performance were reviewed in detail in the first part. The second part of this review covers the use of an IFBR in the biological treatment of different wastewaters through multiple biochemical pathways and how it helped improve performance compared to other reactors. This will enable the researchers to understand the novelty of an IFBR for wastewater treatment and allow them to use it as a potential reactor.

Independent and synergistic bio-reductions of uranium (VI) driven by zerovalent iron in aquifer

Zerovalent iron [Fe(0)] can donate electron for bioprocess, but microbial uranium (VI) [U(VI)] reduction driven by Fe(0) is still poorly understood. In this study, Fe(0) supported U(VI) bio-reduction was steadily achieved in the 160-d continuous-flow biological column. The maximum removal efficiency and capacity of U(VI) were 100% and 46.4 ± 0.52 g/(m³·d) respectively, and the longevity of Fe(0) increased by 3.09 times. U(VI) was reduced to solid UO₂, while Fe(0) was finally oxidized to Fe(III). Autotrophic Thiobacillus achieved U(VI) reduction coupled to Fe(0) oxidation, verified by pure culture. H₂ produced from Fe(0) corrosion was consumed by autotrophic Clostridium for U(VI) reduction. The detected residual organic intermediates were biosynthesized with energy released from Fe(0) oxidation and utilized by heterotrophic Desulfomicrobium, Bacillus and Pseudomonas to reduce U(VI). Metagenomic analysis found the upregulated genes for U(VI) reduction (e.g., dsrA and dsrB) and Fe(II) oxidation (e.g., CYC1 and mtrA). These functional genes were also transcriptionally expressed. Cytochrome c and glutathione responsible for electron transfer also contributed to U(VI) reduction. This study reveals the independent and synergistic pathways for Fe(0)-dependent U(VI) bio-reduction, providing promising remediation strategy for U(VI)-polluted aquifers.

Remediation of the black-odor water body by aquatic plants with plant growth-promoting Rhizobacteria: Lab and pilot tests

To explore an effective, environmental, rapid operating method to repair black and odor water bodies, water samples and sediment samples collected from a polluted municipal lake in Daqing, China, were directly tested in transparent barrels (10 L). Seven groups of optimizing parameters obtained the optimal operating method, and the max removal rate of COD, NH₄⁺-N, NO₃^--N, and TP were achieved (89.18%, 59.65%, 69.50%, and 75.61%) by using aquatic plants with plant growth-promoting Rhizobacteria (PGPR). To further verify the method's effectiveness, lager scale tests were conducted based on a water tank (216 L), and similar removal rates were obtained within 48 h. The water quality index and microbial community structure analysis revealed the mechanisms of the interaction among plants, microorganisms, and pollutants and the main biological processes during water body remediation. Finally, the cost of water body remediation by using this method was estimated.

Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFO_chem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Simultaneous removal of nitrate and ammonium by hydrogen-based partial denitrification coupled with anammox in a membrane biofilm reactor

Hydrogen-based membrane biofilm reactors (MBfRs) are effective for nitrogen removal. However, the safety of hydrogen limited the application of MBfR. Here, a hydrogen-based partial denitrification system coupled with anammox (H₂-PDA) was constructed in an MBfR for reducing hydrogen demand significantly. The metabolomics and structures of microbial communities were analyzed to determine the phenotypic differences and drivers underlying denitrification, anammox, and H₂-PDA. These findings indicated that total nitrogen (TN) removal increased from 57.1% in S1 to 93.7% in S2. During the H₂-PDA process, partial denitrification and anammox contributed to TN removal by 93.7% and 6.3%, respectively. Community analysis indicated that the H₂-PDA system was dominated by the genus Meiothermus, which is involved in partial denitrification. Collectively, these findings confirmed the feasibility of incorporating the H₂-PDA process in a MBfR and form a foundation for the establishment of novel and practical methods for efficient nitrogen removal.

Effects of magnetite on microbially driven nitrate reduction processes in groundwater

Nitrate is a common pollutant in the aquatic environment. Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are the main reduction processes of nitrate. In the relatively closed sediment environment, the competitive interaction of these two nitrate reduction determines whether the ecosystem removes or retains nitrogen. In the process of NO₃^--N bioreduction, Magnetite, which is a common mineral present in soil and other sediments can play a crucial role. However, it is still not clear whether magnetite promotes or inhibits NO₃^--N bioreduction. In this paper, the effect of magnetite on NO₃^--N bioreduction was studied by batch experiments. The results show that magnetite can increase the NO₃^--N reduction rate by 1.48 %, and can inhibit the DNRA process at the beginning of the reaction and then promote the DNRA process. Magnetite changed the microbial community structure in our experiment systems. The relative abundance of Sphingomonas, which mainly exists in a high carbon and low nitrogen environment, increased under sufficient carbon source conditions. The relative abundance of Fe-oxidizing and NO₃^--N reducing bacteria, such as Flavobacterium, increased in the absence of carbon sources but in the presence of magnetite. In addition, magnetite can significantly increase activity of the microbial electron transport system (ETS). the added microbial electronic activity of magnetite increased nearly two-fold under the same experiment conditions. The acid produced by the metabolisms of Pseudomonas and Acinetobacter further promotes the dissolution of magnetite, thus increasing the concentration of Fe (II) in the system, which is beneficial to autotrophic denitrifying bacteria and promote the reduction of NO₃^--N. These findings can enhance our understanding of the interaction mechanism between iron minerals and nitrate reducing bacteria during nitrate reduction under natural conditions.

Use of sponge iron as an indirect electron donor to provide ferrous iron for nitrate-dependent ferrous oxidation processes: Denitrification performance and mechanism

Sponge iron (SI) can serve as an indirect electron donor to provide Fe(II) for the nitrate-dependent ferrous oxidation (NDFO) process, producing OH^- and magnetite. The SI-NDFO system mainly uses Fe(OH)₂ as an electron donor, achieving a TN reduction rate of 0.42 mg-TN/(gVSS·h) for a period of at least 90 days. The enrichment of iron-oxidizing bacteria and the competition of iron-carbon micro-electrolysis for reaction sites on the surface of SI are the main reasons for the improvement of total nitrogen removal efficiency (TNRE). With an influent NO₃^--N concentration of 50 mg/L and a SI concentration of 50 g/L (at pH 5.0 and 30 °C), the TNRE reached a maximum level of 38.28%. In addition, reducing the pH environment was found to improve the denitrification efficiency of the SI-NDFO system, although denitrification stability was also reduced as a result. Overall, the SI-mediated NDFO process is a promising technique.

Biofilm carrier type affects biogenic sulfur-driven denitrification performance and microbial community dynamics in moving-bed biofilm reactors

Autotrophic denitrification with biosulfur (ADBIOS) provides a sustainable technological solution for biological nitrogen removal from wastewater driven by biogenic S⁰, derived from biogas desulfurization. In this study, the effect of different biofilm carriers (conventional AnoxK™ 1 and Z-200 with a pre-defined maximum biofilm thickness) on ADBIOS performance and microbiomics was investigated in duplicate moving bed-biofilm reactors (MBBRs). The MBBRs were operated parallelly in continuous mode for 309 days, whilst gradually decreasing the hydraulic retention time (HRT) from 72 to 21 h, and biosulfur was either pumped in suspension (days 92-223) or supplied in powder form. Highest nitrate removal rates were approximately 225 (±11) mg/L·d and 180 (±7) mg NO₃^--N/L·d in the MBBRs operated with K1 and Z-200 carriers, respectively. Despite having the same protected surface area for biofilm development in each MBBR, the biomass attached onto the K1 carrier was 4.8-fold more than that on the Z-200 carrier, with part of the biogenic S⁰ kept in the biofilm. The microbial communities of K1 and Z-200 biofilms could also be considered similar at cDNA level in terms of abundance (R = 0.953 with p = 0.042). A relatively stable microbial community was formed on K1 carriers, while the active portion of the microbial community varied significantly over time in the MBBRs using Z-200 carriers.

Study of Microbial Cultures for the Bioleaching of Scandium from Alumina Industry By-Products

The disposal of voluminous, highly alkaline, bauxite residue (BR), the industrial by-product of alumina production by the Bayer process, constitutes an intricate global environmental problem. BR, containing valuable metals such as rare-earth elements (REEs)—in particular, scandium (Sc)—can be used as a secondary source for REE extraction. The scope of this study was the investigation of bioleaching as an innovative and environmentally friendly approach for the extraction of Sc from BR. The bioleaching parameters were studied on Greek BR and experiments were performed using different microbial cultures and solid to liquid ratios (S/L). The maximum extraction of Sc was 42% using Acetobacter tropicalis in a one-step bioleaching process at 1% S/L. The main organic acids produced were acetic, oxalic, and citric. The bioleaching data indicated a probable synergistic effect of the different organic acids produced by microorganisms along with a more targeted leaching mechanism.

Improvement of nitrogen removal with iron scraps in floating treatment wetlands

Floating treatment wetland (FTW) in restoration of low C/N ratio wastewater was deemed to a frequently used method. However, the nitrate removal performance in floating beds was limited due to insufficient organic carbon sources. Iron scraps as a potential electron donor was beneficial to the NO₃^--N reduction. To research the removal performance and mechanism of denitrification in FTW with iron scraps, FTW with Iris pseudacorus was built, and iron scraps were added as an electron donor to improve nitrogen removal efficiency. The batch experimental results demonstrated that the proper mass ratio of iron scraps to NO₃^--N was 500:1. With iron scraps, the NO₃^--N removal efficiency of FTW and control system increased significantly to 98.04% and 44.42% respectively in 2 weeks, while there was no obvious influence on the removal of NH₄⁺-N. After adding iron scraps, the proportion of bacteria in the systems related to iron cycle and the relative abundance of nitrifying and denitrifying bacteria have increased obviously. By calculating the nitrogen balance, nitrogen reduction via plant uptake accounted for 8.79%, and the microbial denitrification was the main nitrogen removal pathway in FTW.

Moving bed biofilm reactor developed with special microbial seed for denitrification of high nitrate containing wastewater

Biological denitrification is the most promising alternative approach for the removal of nitrate from wastewater. MBBR inoculated with activated sludge is a widely studied approach, but very few studies have focused on the bioaugmentation of biofilm forming bacteria in MBBR. Our study revealed that the use of special microbial seed of biofilm forming denitrifying bacteria Diaphorobacter sp. R4, Pannonibacter sp. V5, Thauera sp. V9, Pseudomonas sp.V11, and Thauera sp.V14 to form biofilm on carriers enhanced nitrate removal performance of developed MBBR. Various process parameters C/N ratio 0.3, HRT 3 h at Nitrate loading 2400 mg L^-1, Filling ratio 20%, operated with Pall ring carrier were optimized to achieve highest nitrate removal. After 300 days of continuous operation results of whole genome metagenomic studies showed that Thauera spp. were the most dominant and key contributor to the denitrification of nitrate containing wastewater and the reactor was totally conditioned for denitrification. Overall, findings suggest that bench-scale MBBR developed with biofilm forming denitrifying microbial seed accelerated the denitrification process; therefore in conclusion it is suggested as one of the best suitable and effective approach for removal of nitrate from wastewater.

Effect of nickel on the comparative performance of inverse fluidized bed and continuously stirred tank reactors for biogenic sulphur-driven autotrophic denitrification

The comparative performance of an inverse fluidized bed reactor (IFBR) having high density polyethylene beads as carrier materials for biofilm formation and a continuous stirred tank reactor (CSTR), both maintaining autotrophic denitrification using biogenic sulphur (ADBIOS) in the absence and presence of nickel (Ni²⁺), was studied. The reactors were compared in terms of NO₃^--N and NO₂^--N removal and SO₄^2--S production throughout the study. A simulated wastewater with an inlet NO₃^--N concentration of 225 mg/L and a decreasing concentration of biogenic sulphur (bio-S) from 1.5 to 0.375 g/L was used. Both reactors were operated at a hydraulic retention time (HRT) of 48 h for 140 days and at an HRT of 42 h for the following 68 days. A more efficient ADBIOS was observed in the CSTR than IFBR throughout the study due to a better mixing of the feed wastewater in the bulk liquid and a higher availability of bio-S to the suspended cells. The NO₃^--N removal efficiency in the IFBR decreased by approximately 41% when the feed bio-S was reduced to 0.375 g/L, while it remained unaffected in the CSTR. Conversely, the presence of Ni²⁺ did not significantly affect NO₃^--N removal in both reactors even at a feed Ni²⁺ concentration of 120 mg/L. The highest NO₃^--N removal rates achieved were 86 and 108 mg NO₃^--N/(L·day) in the IFBR and CSTR, respectively, in the presence of 120 mg/L of feed Ni²⁺ at an HRT of 42 h. Batch studies conducted with acclimatized biomass showed that the continuous-flow operation mode in both reactors played a major role in helping the autotrophic denitrifiers to tolerate Ni²⁺ toxicity.

Promotion of Microbial Oxidation of Structural Fe(II) in Nontronite by Oxalate and NTA

Iron redox cycling occurs extensively in soils and sediments. Previous research has focused on microbially mediated redox cycling of aqueous Fe. At circumneutral pH, most Fe occurs in solid phase, where Fe and organic ligands interact closely. However, the role of organic ligands in microbial oxidation of solid-phase Fe(II) is not well understood. Here, we incubated reduced nontronite NAu-2 (rNAu-2) with an iron-oxidizing bacterium and in the presence of oxalate and nitrilotriacetic acid. These ligands significantly enhanced the rate and extent of microbial oxidation of structural Fe(II) in rNAu-2. Aqueous and solid-phase analyses, coupled with biogeochemical modeling, revealed a pathway for ligand-enhanced bio-oxidation of solid-phase Fe(II): (1) dissolution of rNAu-2 to form aqueous Fe(II)-ligand complex; (2) bio-oxidation to Fe(III)-ligand complex; (3) rapid reduction of Fe(III)-ligand complex to Fe(II)-ligand complex by structural Fe(II) in rNAu-2. In this process, the Fe(II)-ligand and Fe(III)-ligand complexes effectively serve as electron shuttle to expand the bioavailable pool of solid-phase Fe(II). These results have important implications for a better understanding of the bioavailability and reactivity of solid-phase Fe pool in the environment.

Improvement of ferrous ion-dependent nitrate removal (FeNiR) process with chelating ferrous ion as substrate

In order to improve the ferrous ion-dependent nitrate removal (FeNiR) process, hexametaphosphate chelated ferrous ion was used as substrate to replace the free ferrous ion. With hexametaphosphate chelated ferrous ion as substrate, the influent pH was adjusted to 6.8, and as a result a higher effluent pH (7.2) was detected. The volumetric removal rate (VRR) of nitrate kept at 0.42 ± 0.03 kg-N/(m³∙d) for 48 days and the corresponding nitrogen removal efficiency was 94.39 ± 4.57%. After 88 days of cultivation, FeNiR granules became small because of the oligotrophic substrate. The transmission electron microscope (TEM) analysis showed that less iron encrustation was formed on the surface or in the periplasm of FeNiR cells. The linear curve of the living cell percentage versus time showed that the death rate of FeNiR cells with chelated ferrous ion as substrate was much lower than that with free ferrous ion as substrate (0.4210 vs 0.9221). Without iron encrustation, both the FeNiR activity and alkaline phosphatase (ALP) activity of FeNiR cells kept at high level and thus the efficiency of the FeNiR reactor kept stable and high. With hexametaphosphate chelated ferrous ion as substrate, the pH in bulk liquid was high (pH = 7.2) resulting in the high FeNiR rate, and less iron encrustation was formed around cells ensuring the stability of high FeNiR rate. Therefore, using hexametaphosphate chelated ferrous ion as substrate was an efficient way to improve the FeNiR process.

A novel strategy for accelerating the recovery of a Fe(II)-inhibited anammox reactor by intermittent addition of betaine: Performance, kinetics and statistical analysis

In this manuscript, Fe(II) inhibition of anammox and its recovery were investigated, and the performance, kinetics and statistical features were comprehensively studied simultaneously. Anammox was suppressed and completely inhibited by the addition of 109.29 and 378.57 mg/L Fe(II), respectively, via uncompetitive inhibition. Nitrite inhibition of anammox was best fitted by the Edwards model and Aiba model. EDTA-2Na wash (0.5, 1.0, 1.5, and 2.0 mM) had a limited effect on anammox recovery, while the addition of 2.0 mM betaine accelerated anammox recovery. Prolonged betaine addition caused an unintended reduction of anammox activity, though it self-recovered after the withdrawal of betaine. The modified Boltzmann model most accurately simulated the processes of anammox recovery using the EDTA-2Na wash, betaine regulation and self-recovery, and the modified Stover-Kincannon model was able to assess the results of anammox recovery. The one-sample t-test was successfully applied to determine the effects of these three recovery strategies on inhibited anammox, which were short-term disinhibition or long-term recovery effects. The above-mentioned results demonstrate that an intermittent addition of betaine, which is a better alternative to frequently-used but poorly-degradable EDTA, may be a useful and environmentally friendly recovery strategy for Fe(II)-inhibited anammox reactor.

Enhanced long-term advanced denitrogenation from nitrate wastewater by anammox consortia: Dissimilatory nitrate reduction to ammonium (DNRA) coupling with anammox in an upflow biofilter reactor equipped with EDTA-2Na/Fe(II) ratio and pH control

A long-term experiment in an anaerobic ammonium oxidation (anammox) reactor showed that anammox consortia could perform a stable and efficient Fe(II)-dependent dissimilatory nitrate reduction to ammonium (DNRA) coupled to the anammox (DNRA-anammox) process by controlling the EDTA-2Na/Fe(II) ratio and pH, with a total nitrogen removal rate (TNRR) of 0.23 ± 0.01 kg-N/m3/d. Anammox bacteria (Candidatus Kuenenia) were the dominant and functional microbes in such a nitrate wastewater treatment system. Visual MINTEQ analysis showed that the EDTA-2Na/Fe(II) molar ratio affected the influent composition of Fe and EDTA species and hence nitrate removal, while pH influenced both nitrate removal and the coupling degree of the Fe(II)-dependent DNRA-anammox process due to its own physiology. The kinetic simulation results showed that excess EDTA-2Na imposed a competitive inhibition on the Fe(II)-dependent DNRA-anammox process, and the Bell-shaped (A), (B), (C) and Ratkowsky models could be used to explore the pH dependency of the Fe(II)-dependent DNRA-anammox process.

Comparing performances, costs and energy balance of ex situ remediation processes for PAH-contaminated marine sediments

This study proposes a comparison of different ex situ technologies aimed at the removal of polycyclic aromatic hydrocarbons from marine sediments in terms of performances, costs and energy balance. In accordance with the principles of water-energy nexus, anaerobic bioremediation, soil washing and thermal desorption were investigated under low liquid phase and temperature conditions using phenanthrene (PHE) as model compound. After 42 days of anaerobic bioremediation, the highest PHE biodegradation of 68 and 64% was observed under denitrifying and methanogenic conditions, respectively, accompanied by N₂ and CH₄ production and volatile fatty acid accumulation. During soil washing, more than 97% of PHE was removed after 60 min using a solid-to-liquid ratio of 1:3. Along the same treatment time, low-temperature thermal desorption (LTTD) allowed a PHE removal of 88% at 200 °C. The economic analysis indicated that LTTD resulted in a higher cost (i.e. 1782 € m^-3) than bioremediation and soil washing (228 and 371 € m^-3, respectively). The energy balance also suggested that bioremediation and soil washing are more sustainable technologies as a lower required energy (i.e. 16 and 14 kWh m^-3, respectively) than LTTD (i.e. 417 kWh m^-3) is needed.

Removal of polycyclic aromatic hydrocarbons during anaerobic biostimulation of marine sediments

This study proposes the supplementation of digestate, fresh organic fraction of municipal solid waste (OFMSW) and a nutrient solution during the anaerobic biostimulation of marine sediments contaminated by polycyclic aromatic hydrocarbons (PAHs). The experimental activity was conducted with four PAHs (i.e. phenanthrene, anthracene, fluoranthene and pyrene) under controlled mesophilic conditions (37 ± 1 °C) in 100 mL serum bottles maintained at 130 rpm. After 120 days of incubation, the highest total PAH degradation of 53 and 55% was observed in the experiments with digestate + nutrients and OFMSW + nutrients, respectively. Phenanthrene was the most degraded PAH and the highest removal of 69% was achieved with OFMSW + nutrients. The anaerobic PAH degradation proceeded through the accumulation of volatile fatty acids and the production of hydrogen and methane as biogas constituents. The highest cumulative biohydrogen production of 80 mL H₂·g VS^-1 was obtained when OFMSW was used as the sole amendment, whereas the highest biomethane yield of 140 mL CH₄·g VS^-1 was obtained with OFMSW + nutrients. The evolution of PAH removal during anaerobic digestion revealed a higher impact of the methanogenic phase rather than acidogenic phase on PAH degradation.

Biphasic effect of nitrate on anaerobic ammonium oxidation (anammox) and related kinetic modeling

Nitrate is a byproduct of the anaerobic ammonium oxidation (anammox) process and is related to its electron transfer. However, little is known about the influence of nitrate on the anammox process. In this work, the biphasic effect of exogenous nitrate on the anammox process was investigated in an upflow biofilter (UBF) reactor with ammonium as the sole electron donor. The responses of anammox to increased nitrate were analyzed by one-way ANOVA test and found to be significantly different under a constant and decreased nitrite condition (p < 0.01). With a single increase in nitrate and constant ammonium and nitrite in the influent, the total nitrogen removal rate (TNRR) of anammox was uninhibited, but stoichiometry deviated and nitrate production always showed a linear decrease. In contrast, anammox exhibited a range of activity with constant ammonium and simultaneously increased nitrate and decreased nitrite in the influent, including a continuous reduction of TNRR, a nonpersistent ammonium overconsumption and a pronounced nonlinear response of nitrate production. Correlation analysis shows that the lack of ammonium overconsumption was accompanied by the disappearance of nitrate underproduction. Kinetic models of product formation were effectively used to explore the nitrate production behavior of anammox subjected to increased nitrate, and the metabolite of nitrate was divided into a growth negative coupling type and growth (partial) coupling type under a constant and decreased nitrite condition, respectively. These findings collectively suggest that nitrate has a biphasic effect on the anammox process and is correlated with the availability of nitrite.

Mineral characterization of the biogenic Fe(III)(hydr)oxides produced during Fe(II)-driven denitrification with Cu, Ni and Zn

The recovery of iron and other heavy metals by the formation of Fe(III) (hydr)oxides is an important application of microbially-driven processes. The mineral characterization of the precipitates formed during Fe(II)-mediated autotrophic denitrification with and without the addition of Cu, Ni, and Zn by four different microbial cultures was investigated by X-ray fluorescence (XRF), Raman spectroscopy, scanning electron microscopy equipped with energy dispersive X-Ray analyzer (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray Powder Diffraction (XRD) analyses. Fe(II)-mediated autotrophic denitrification resulted in the formation of a mixture of Fe(III) (hydr)oxides composed of amorphous phase, poorly crystalline (ferrihydrite) and crystalline phases (hematite, akaganeite and maghemite). The use of a Thiobacillus-dominated mixed culture enhanced the formation of akaganeite, while activated sludge enrichment and the two pure cultures of T. denitrificans and Pseudogulbenkiania strain 2002 mainly resulted in the formation of maghemite. The addition of Cu, Ni and Zn led to similar Fe(III) (hydr)oxides precipitates, probably due to the low metal concentrations. However, supplementing Ni and Zn slightly stimulated the formation of maghemite. A thermal post-treatment performed at 650 °C enhanced the crystallinity of the precipitates and favored the formation of hematite and some other crystalline forms of Fe associated with P, Na and Ca.

Effect of Fe(II) on reactivity of heterotrophic denitrifiers in the remediation of nitrate- and Fe(II)-contaminated groundwater

Heterotrophic denitrifiers, capable of simultaneous nitrate reduction and Fe(II) oxidation, can be applied for the remediation of nitrate and Fe(II) combined contamination in groundwater. Under strictly anaerobic condition, denitrifying microbial communities were enriched with the coexistence of soluble nitrate, Fe(II) and associated nutrient elements to monitor the denitrification process. Low abundance of Fe(II) (e.g., 10 mg L^-1 in this study) tended to stimulate the activity of denitrifying microbial communities. However, elevated Fe(II) concentration (50 and 100 mg L^-1 in this study), acted as a stress, strongly inhibited the activity and reproduction of denitrifiers. Besides, through thermodynamics calculations, methanol rather than Fe(II) was proved to be the preferable electron donors for both energy metabolism and anabolism. Betaproteobacteria was found to be the most predominant (sub)phylum in all enriched microbial assemblages. Methylovesartilis was the most predominant group mainly catalyzed for methanol based denitrification, and others denitrifiers included Methylophilaceae, Dechloromonas and Denitratisoma. Excessive Fe(II) in the solution greatly reduced the proportions of these denitrifying groups, while the influence seemed to be less apparent on functional genes composition. As such, a conceptional metabolism pathway of the most dominant genus (i.e., Methylovesartilis) for nitrate reducing as well as methanol and Fe(II) oxidation confirmed that biotic nitrate reducing and Fe(II) oxidizing were potentially proceeded in cytoplasm by enzymes such as NarGHI. The Fe(II) oxidation rate depended on the rate of Fe(II) entering into the cell. These findings provide a clear mechanistic understanding of heterotrophic denitrification coupling with Fe(II) oxidation, and environmental implication for the bioremediation of nitrate and Fe(II) contaminated groundwater.

Biokinetics of microbial consortia using biogenic sulfur as a novel electron donor for sustainable denitrification

In this study, the biokinetics of autotrophic denitrification with biogenic S⁰ (ADBIOS) for the treatment of nitrogen pollution in wastewaters were investigated. The used biogenic S⁰, a by-product of gas desulfurization, was an elemental microcrystalline orthorhombic sulfur with a median size of 4.69 µm and a specific surface area of 3.38 m²/g, which made S⁰ particularly reactive and bioavailable. During denitritation, the biomass enriched on nitrite (NO₂^-) was capable of degrading up to 240 mg/l NO₂^--N with a denitritation activity of 339.5 mg NO₂^--N/g VSS·d. The use of biogenic S⁰ induced a low NO₂^--N accumulation, hindering the NO₂^--N negative impact on the denitrifying consortia and resulting in a specific denitrification activity of 223.0 mg NO₃^--N/g VSS·d. Besides Thiobacillus being the most abundant genus, Moheibacter and Thermomonas were predominantly selected for denitrification and denitritation, respectively.

Denitrifier communities impacted by heavy metal contamination in freshwater sediment

Heavy metals are widely detected in natural environments, however their impacts on denitrifier community in freshwater ecosystem remain unclear. The present study investigated the changes of denitrifier communities (based on nosZ (nitrous oxide reductase) gene) in a freshwater reservoir contaminated by a severe accidental spill of heavy metals. The abundance of nosZ-denitrifiers drastically decreased, and their community richness, diversity and structure also showed considerable variations. The mainly detected denitrifying bacteria included Pseudogulbenkiania, Pseudomonas and two unknown groups. These major nosZ-denitrifier groups responded in different ways to heavy metal pollution. Metal contamination could exert a profound influence on denitrifier community in freshwater sediment. This work could provide some new insights to the impact of metal pollution on nitrogen cycling.

Role and application of iron in water treatment for nitrogen removal: A review

It is crucial to have a review on the role of iron in water treatment for the guidance towards the selection of appropriate processes, content of iron, and application conditions, as there are few reviews available at present and the systematic information is lacking for both researchers and engineers. The objectives of this review are to summarize the state of arts with respect to iron applied in nitrogen removal, discuss chemical and biological or bio-chemical combined nitrogen removal pathways and processes coupled with iron, and to reveal reaction mechanisms as well as providing references or even solutions to pertinent the practical engineering application of nitrate removal coupling with iron. The following information have been summarized and discussed in details: (1) iron based materials with varieties of preparations and forms, (2) major coupling ways of nitrogen removal methods or processes with iron application, (3) chemical reaction equations about a variety of chemical and biological or bio-chemical combined processes and the main mechanisms. In addition, challenges and/or drawbacks during the nitrogen removal processes will also be discussed in this paper, which is aimed to seek better practical engineering applications of nitrate removal coupling with iron.

Effect of Cu, Ni and Zn on Fe(II)-driven autotrophic denitrification

Fe(II)-mediated autotrophic denitrification in the presence of copper (Cu), nickel (Ni) and zinc (Zn) with four different microbial cultures was investigated in batch bioassays. In the absence of metals, complete nitrate removal and Fe(II) oxidation were achieved with a Thiobacillus-dominated mixed culture and Pseudogulbenkiania sp. 2002 after 7 d. A nitrate removal of 96 and 91% was observed with a pure culture of T. denitrificans and an activated sludge enrichment, respectively, after 10 d of incubation. Cu, Ni and Zn were then supplemented at an initial concentration of 5, 10, 20 and 40 mg Me/L. A decrease of approximately 50% of the soluble metal concentrations occurred in the first 4 d of denitrification, due to metal precipitation, co-precipitation, sorption onto iron (hydr)oxides, and probably sorption onto biomass. A higher sensitivity to metal toxicity was observed for the microbial pure cultures. Pseudogulbenkiania sp. 2002 was the least tolerant among the biomasses tested, resulting in only 6, 8 and 6% nitrate removal for the highest Cu, Ni and Zn concentrations, respectively. In contrast, the highest nitrate removal efficiency and specific rates were achieved with the Thiobacillus-dominated mixed culture, which better tolerated the presence of metals. Averagely, Cu resulted in the highest inhibition of nitrate removal, followed by Zn and Ni.