Effects of quinoid redox mediators on the activity of anammox biomass

Tracking the nitrogen transformation in saline wastewater by marine anammox bacteria-based Fe(II)-driven autotrophic denitratation and anammox

Marine anammox bacteria-based Fe(II)-driven autotrophic denitratation and anammox (MFeADA) was investigated for nitrogen removal from saline wastewater for the first time. The study demonstrated that varying influent doses of Fe(II), which participate in the Fe cycle, significantly influenced nitrogen removal performance by altering the fate of nitrite. When 50 mg/L Fe(II) was added, the nitrogen removal was mainly performed by the anammox and Fe(II)-driven autotrophic denitratation (FeAD). As the Fe(II) rose to 100-150 mg/L, the anammox, FeAD and Feammox mainly occurred. Optimal nitrogen removal efficiency, reaching 93 %, was achieved at an influent Fe(II) concentration of 150 mg/L. As the Fe(II) reached 250 mg/L, however, nitrate was directly reduced to dinitrogen gas by the excessive Fe(II) through the Fe(II)-driven autotrophic denitrification (FeADN). Candidatus Scalindua (4.1 %), Marinicella (5.3 %) and SM1A02 (31.8 %) were the dominant functional microbes. In addition, the normalized nitrate reductase abundance was about 3.1 times that of nitrite reductase, leading to the occurrence of FeAD, which achieved a stable nitrite supply for marine anammox bacteria. This novel study can promote the practical implementation of the MFeADA process in nitrogen-laden saline wastewater treatment.

Enhanced nitrogen removal of the anaerobic ammonia oxidation process by coupling with an efficient nitrate reducing bacterium (Bacillus velezensis M3-1)

Bacillus velezensis M3-1 strain isolated from the sediment of Myriophyllum aquatium constructed wetlands was found to efficiently convert NO3--N to NO2--N, and the requirements for carbon source addition were not very rigorous. This work demonstrates, for the first time, the feasibility of using the synergy of anammox and Bacillus velezensis M3-1 microorganisms for nitrogen removal. In this study, the possibility of M3-1 that converted NO3--N produced by anammox to NO2--N was verified in an anaerobic reactor. The NO3--N reduction ability of M3-1 and denitrifying bacteria in coupling system was investigated under different C/N conditions, and it was found that M3-1 used carbon sources preferentially over denitrifying bacteria. By adjusting the ratio of NH4+-N to NO2--N, it was found that the NO2--N converted from NO3--N by M3-1 participated in the original anammox.The nitrogen removal efficacy (NRE) of the coupled system was increased by 12.1%, compared to the control group anammox system at C/N = 2:1. Functional gene indicated that it might be a nitrate reducing bacterium.This study shows that the nitrate reduction rate achieved by the Bacillus velezensis M3-1 can be high enough for removing nitrate produced by anammox process, which would enable improve nitrogen removal from wastewater.

Enhancing the nitrogen removal of anammox sludge by setting up novel redox mediators-mediated anammox process

Anaerobic ammonium oxidizing (anammox) bacteria have been proven weak-electroactive. However, the impact of exogenous anthraquinone-2,6-disulfonate (AQDS) on the anammox activity, although it usually plays essential roles in the life activities of many other electroactive microorganisms, is still unknown. Therefore, this study further explored the influences of AQDS on the anammox activity and the interaction mechanism with anammox bacteria, as well as the behaviors of NH4+, NO2-, and NO3-. The results showed that exogenous AQDS increased the ammonium and total nitrogen removal rates by 12.8% and 10.7%, respectively. Interestingly, the conversion from NO2- to NO3- was significantly reduced after adding AQDS, resulting in a 40.1% reduction in NO3- production of anammox process. In this study, we found for the first time that anammox bacteria could not only carry out the conventional anammox process but also perform a weak redox mediator-mediated anammox process, which could achieve the 1:1 consumption of NH4+ and NO2-. The redox mediator-mediated anammox process was related to an endogenous redox mediator (ERM) synthesized and secreted by anammox bacteria, whose redox midpoint potential was around -0.26 V (vs. Ag/AgCl). After adding AQDS, not only the ERM-mediated anammox process was enhanced, but also two novel redox mediator-mediated anammox processes were introduced, including the AQDS-mediated anammox process and ERM-AQDS-mediated anammox process. These three redox mediator-mediated anammox processes significantly improved the nitrogen removal performance of anammox bacteria and reduced energy consumption. These findings will help reduce the dependence of anammox technology on NO2-, reduce the cost of subsequent treatment of NO3-, and provide new visions for optimizing and applying anammox technology.

Anammox with alternative electron acceptors: perspectives for nitrogen removal from wastewaters

In the context of the anaerobic ammonium oxidation process (anammox), great scientific advances have been made over the past two decades, making anammox a consolidated technology widely used worldwide for nitrogen removal from wastewaters. This review provides a detailed and comprehensive description of the anammox process, the microorganisms involved and their metabolism. In addition, recent research on the application of the anammox process with alternative electron acceptors is described, highlighting the biochemical reactions involved, its advantages and potential applications for specific wastewaters. An updated description is also given of studies reporting the ability of microorganisms to couple the anammox process to extracellular electron transfer to insoluble electron acceptors; particularly iron, carbon-based materials and electrodes in bioelectrochemical systems (BES). The latter, also referred to as anodic anammox, is a promising strategy to combine the ammonium removal from wastewater with bioelectricity production, which is discussed here in terms of its efficiency, economic feasibility, and energetic aspects. Therefore, the information provided in this review is relevant for future applications.

The role of anthraquinone-2-sulfonate on intra/extracellular electron transfer of anaerobic nitrate reduction

To improve the electron (e^-) transfer efficiency, exogenous redox mediators (RMs) were usually employed to enhance the denitrification efficiency due to the electron shuttling. Previous studies were mainly focused on how to improve the extracellular electron transfer (EET) by exogenous RMs. However, the intracellular electron transfer (IET), another crucial e^- transfer pathway, of biological denitrification was scarcely reported, especially for the relationship between the denitrification and IET. In this study, Coenzyme Q, Complexes I, II and III were determined as the core components in the IET chain of denitrification by using four specific respiration chain inhibitors (RCIs). Anthraquinone-2-sulfonate (AQS) partially recovered the IET of denitrification from NO₃^--N to N₂ gas when the RCIs were added. Specifically, the generations of N₂ gas were improved by 9.68%-18.25% in the experiments with RCIs and AQS, comparing to that with RCIs. nrfA gene was not detected by reverse transcription-polymerase chain reaction, suggesting that Klebsiella oxytoca strain could not conduct dissimilatory nitrate reduction to ammonium. Nitrate assimilation was considered as the main NH₄⁺-N formation way of K. oxytoca strain. The two e^- transfer pathways of denitrification were constructed and the roles of AQS on the IET and EET of denitrification were specifically discussed. The results of this study provided a better understanding of the e^- transfer pathways of denitrification, and suggested a potential practical use of exogenous RM on bio-treatment of nitrate-containing wastewater.

Promotion of anammox process by different graphene-based materials: Roles of particle size and oxidation degree

Graphene oxide (GO) and reduced graphene oxide (RGO) have been applied in the anaerobic ammonium oxidation (anammox) process for nitrogen removal as electron shuttles. However, there is still controversy about their efficacy. In this study, nine graphene-based materials with a gradient of three particle sizes (large (l), medium (m) and small (s) sizes) and oxidation degrees, were used to compare their effects on the anammox process efficiency. The graphene-based materials include GO and its reduced products (RGO250 and RGO800) obtained at temperatures of 250 °C and 800 °C respectively. It was observed that their enhancements on the anammox process were in the order of GO > RGO800 > RGO250. In detail, at the dose of 100 mg/L, specific anammox activities (SAA) were promoted by 6.7% (l-GO), 4.9% (l-RGO800), 11.5% (m-GO), 7.3% (m-RGO800), 13.2% (s-GO) and 8.3% (s-RGO800) compared to the control respectively; while RGO250 with the same dose inhibited the process. In addition, the enhancement of the anammox process was increasing with the decreasing size of GO and RGO800. The nitrite reductase (NIR) activity was greatly increased by up to 24.9% with the presence of GO, which might be attributed to organized and specific electron transport with oxygen functional groups. The finding of hydroxyl on RGO and increasing content of oxygen determined after reaction detected by Fourier transform infrared spectroscopy and energy dispersive spectrometer respectively, indicated the essential condition for RGO's function on transferring electrons for key enzymes in annamox bacteria. Most importantly, O/C (Oxygen/Carbon) ratios of graphene-based materials had greater effects on the promotion of the anammox process than the particle size and electrical conductivity.

The feasibility and mechanism of redox-active biochar for promoting anammox performance

Redox-active biochar has been regarded as an effective additive to promote heterotrophic denitrification, yet little is known about the feasibility of adding biochar for promoting anammox performance. In this study, we investigated the effects of different biochar doses (3-14 g/L; 1.5-7.1 g/g VSS) on anammox performance. Results showed that, in a short term (40 days), biochar could enhance anammox nitrogen removal rate (NRR) and nitrogen removal efficiency (NRE) by 0-18.0% and 0.2%-11.6%, respectively; this enhancement effect increased at 3-10 g biochar/L assays and reached a plateau at 10-14 g biochar/L assays. The optimal biochar dosage was identified to be 10 g/L (5.1 g/g VSS), with the NRR and NRE being 5.6%-18.0% and 4.0%-11.6% higher than those of the control, respectively. The highest specific anammox activity was simultaneously obtained at 10 g biochar/L assay, being 51% higher than that of the control. It revealed that biochar promoted the secretion of extracellular polymeric substances (increased by 30%-40% compared with that of the control) and increased the ratio of extracellular proteins to polysaccharides as well, directly enhancing the extracellular electron transfer capacity (ETC) of anammox biomass. The increased ETC of anammox biomass would further accelerate the metabolic activities of anammox bacteria, and promote the relative abundance of anammox bacteria, i.e., Ca. Brocadia was enriched by 5.8-12.6 folds than that of the control. These results demonstrate that biochar is feasible to enhance anammox activity and nitrogen removal performance, facilitating to a fast startup and enhanced nitrogen removal of anammox system.

Nitrogen removal mechanism of marine anammox bacteria treating nitrogen-laden saline wastewater in response to ultraviolet (UV) irradiation: High UV tolerance and microbial community shift

Salt stress can be naturally overcome by marine anammox bacteria (MAB), while their low growth rate and sensitivity to operational conditions are still challenges for the application of anammox. To enhance the enrichment of MAB and decipher the effects of ultraviolet (UV) irradiation on MAB, UV was introduced in the nitrogen removal of MAB treating nitrogen-laden saline wastewater for the first time. The results indicated that MAB were resistant to a fairly high UV-C dose, 12000 mJ/cm². Their relative abundance was enhanced by 1.2 folds under 12000 mJ/cm² UV-C. However, the relative abundance of Actinobacteria, Acidobacteria, Chloroflexi and Marinicella were greatly dropped with enhanced UV-C dose. The tolerance mechanism was diversified, e.g. excessive extracellular polymeric substances, special structure of MAB and interspecific competition/cooperation. Although further study was still needed, the findings shed a light on MAB enrichment and exploited great potentials of MAB in nitrogen-laden saline wastewater treatment.

Enhancement of nitrite production via addition of hydroxylamine to partial denitrification (PD) biomass: Functional genes dynamics and enzymatic activities

This study investigated the activity of partial denitrification (PD) biomass/key enzymes, functional gene expressions in response to 0 ~ 50 mg/L hydroxylamine (NH₂OH) addition. Results indicated that NH₂OH contributed to nitrite (NO₂^--N) production, facilitating the maximum increase of nitrate (NO₃^--N) to NO₂^--N transformation ratio to 80.47 ± 2.82%, leading to 2.56-fold NO₂^--N higher than those of control. The observed transient inhibitory effect on NO₃^--N reduction process was attributed by high-level NH₂OH (35 ~ 50 mg/L). Enzymatic assays revealed the enhanced activity of both NO₃^--N and NO₂^--N reductase while the former showed obvious superiority which led to high NO₂^--N accumulation. These results were further confirmed by the corresponding functional genes (narG, napA, nirS and nirK). Besides, negative influence of NH₂OH addition was limited to PD aggregates, due to the increasing secretion of extracellular polymeric substances (EPS) as well as proteins/polysaccharides ratios in tightly-bound structure of EPS.

Perchlorate bioreduction by anaerobic granular sludge immobilised with Fe-HA complex: Performance, extracellular polymeric substances and microbial community structure

An iron-humic acid (Fe-HA) complex was used as a redox mediator in perchlorate (ClO₄^-) bioreduction. Bioreduction performance, extracellular polymeric substances (EPS), and microbial community structure were comprehensively explored using different types of anaerobic granular sludge (AnGS) immobilised without the Fe-HA complex (AnGS_CON) and with the Fe-HA complex (AnGS_FH). The ClO₄^- was completely removed by AnGS_CON by day 20, while the ClO₄^- was completely removed by AnGS_FH by day 6. The AnGS immobilised with the Fe-HA complex significantly increased the ClO₄^- bioreduction. The acceleration of ClO₄^- bioreduction was also explained by the mixed liquor volatile suspended solids (MLVSS), MLVSS/mixed liquor suspended solids (MLSS), EPS composition, and microbial community structure. Compared with AnGS_CON, the MLVSS and MLVSS/MLSS of the AnGS_FH increased 1.4- and 1.2-fold, respectively. Humic substances in the EPS were stimulated by the Fe-HA complex. The microbial community structure analysis indicated that perchlorate and quinone reducing bacteria were enriched by the Fe-HA complex. Based on the analysis, the ClO₄^- bioreduction mechanism of the AnGS_FH was revealed because the Fe-HA complex in the AnGS increased the biomass concentration, biological activity, and redox-active mediator and shifted the microbial community structure.

Response of nitrogen removal performance, functional genes abundances and N-acyl-homoserine lactones release to carminic acid of anammox biomass

Carminic acid (CA) can serve as a redox mediator and influence the electron transfer process. CA dosages of 0-5 mg/L were added to anaerobic ammonia oxidation (anammox) biomass. The results illustrated that CA not only reduced the inorganic nitrogen removal efficiency, but also decreased the nitrogen removal rate. The deterioration of nitrogen removal performance was due to the excess production of nitrate-nitrogen. The concentration of extracellular polymeric substances showed a decrease together with a decline in N-acyl-homoserine lactones release. CA addition decreased the activity of anammox bacteria while increasing the nitrifying potential. Quantitative reverse transcription polymerase chain reaction showed a decrease in anammox functional genes (nirS, hzo, and hzsB) and promotion of the expression of the nxrB gene, which corresponded with a decrease in anammox bacteria activity and the improvement of nitrifying potential. As a result, CA should not be added to anammox biomass.

Insight into the short-term effect of fulvic acid on nitrogen removal performance and N-acylated-L-homoserine lactones (AHLs) release in the anammox system

Fulvic acid (FA) can serve as electron shuttles between bacteria and electron acceptors. It explored the short-term effect of FA dose on nitrogen removal performance and N-acylated-_L-homoserine lactones (AHLs) release change in the anaerobic ammonium oxidation (anammox) system. The results demonstrated that the total inorganic nitrogen removal efficiency increased with the FA dosages from 0.5 mM to 1 mM. FA addition improved anammox bacteria activity, together with extracellular polymeric substances production. FA addition from 0.5 mM to 1 mM stimulated AHLs release in both water and biomass phases, which indicated that the quorum sensing could be improved. These findings revealed that the addition of FA could improve quorum sensing and then enhance nitrogen removal performance.

Immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated. Three redox mediators (RMs), namely, 2-methyl-1,4-naphthoquinone (ME), anthraquinone (AQ) and 1-dichloroanthraquinone (1-AQ) were catalyzed to reduce nitrate to only nitrite by denitrification to integrate with the anammox process for nitrogen removal. First, our experimental results showed that there were 35.8, 42.2 and 53.0 mg-N L⁻¹ nitrite accumulation values with the addition of ME, AQ and 1-AQ, respectively, at the dose of 75 µM by the denitrification process at C/N = 2, which were 25.6%, 48.2% and 86.1% higher than that of the control without the addition of any RMs. Nitrate reductase activities were higher than that of nitrite reductase affected by RMs, which was the main reason for nitrite accumulation and further maintenance of the anammox process. Second, owing to the stable nitrite production by the partial denitrifying biomass with the addition of 1-AQ, the nitrogen removal rate of the reactor that integrated the partial denitrification and anammox process reached 1788.36 g-N m⁻³ d⁻¹ only using ammonia and nitrate as the influent nitrogen resource in the long-term operation. Third, the 16S rDNA sequencing results demonstrated that Yersinia frederiksenii and Thauera were the primary groups of the denitrifying biomass, which were considered the dominant partial denitrification species.

In this study, immobilizing partial denitrification biomass and redox mediators to integrate with the anammox process for nitrogen removal was investigated.

Spontaneous changes in dissolved organic matter affect the bio-removal of steroid estrogens

Microbial action is the main pathway removing steroid estrogens (SEs) from both aerobic and anaerobic natural waters. The rate is influenced by other active substances present, particularly dissolved organic matter (DOM). DOM in natural surface waters has unstable components which undergo spontaneous photochemical oxidation, biological oxidation, chemical oxidation changes. How these changes influence the biosorption and bio-removal of SEs was the subject of this research. Photo oxidation-induced DOM increased the proportion of the fluorescence in area V, but biological oxidation and chemical oxidation caused fluorescence area V to decrease. All three oxidation processes can reduce the proportions of molecular weight (MW) > 5 kg·mol^-1 and increase the proportions of MW < 5 kg·mol^-1. Both the electron transfer capacity decreased monotonically with photo oxidation and chemical oxidation ageing, but biological oxidation ageing increased them. 17β-estradiol (E2) was the SEs used in the experiments. In aerobic conditions, fresh river humic acids (RHA) and aged RHA had the stronger mediating effect on the rate of E2 bio-removal under aerobic conditions. Its greater effectiveness was probably related to its binding with E2. Binding, biosorption of E2 and bio-removal of E2 were strongly positively correlated with the elemental C (R > 0.8, p ≤ 0.01) and SUVA₂₅₄ (R > 0.8, p ≤ 0.01) by correlation matrix. Besides, fresh river fulvic acids (RFA) and aged RFA had the bigger mediating effect on E2 bio-removal under anaerobic conditions, and this imply that changes in aged DOM affected by other electron transfer groups in an anaerobic water environment. In anaerobic conditions, biosorption of E2 and binding action could cluster together with SUVA₂₅₄, p(v), and 1 kg·mol^-1 < MW < 5 kg·mol^-1 by redundancy analysis, and but bio-removal of E2 could be well polymerized with EAC, EDC, p(iv), and MW > 5 kg·mol^-1.

Cold anammox process and reduced graphene oxide - Varieties of effects during long-term interaction

Because of its energy efficiency, the anaerobic ammonium oxidation (anammox) process has been recognized as the most promising biological nitrogen removal process, but its implementation in mainstream wastewater treatment plants is limited by its relatively high optimal temperature (30 °C). Recently, it was shown that during short-term batch experiments, reduced graphene oxide (RGO) displayed accelerated reaction activity at low temperatures (10-15 °C). In this study, the long-term effects of RGO on the low-temperature anammox process in a sequencing batch reactor (SBR), are studied for the first time, including different methods of interaction. The results presented here show that RGO can stimulate anammox activity up to 17% through two factors: bacterial growth stimulation, which was especially significant at higher temperatures (>15 °C), and an increase of the anammox reaction rate, which occurred only below 15 °C. The bacterial community structure was not influenced by addition of RGO. Moreover, after incubation in an anammox bioreactor, RGO showed signs of degradation and chemical changes as evidenced by the presence of oxygen and calcium on its surface. According to the literature and the obtained results, it is proposed that RGO is oxidized and oxygen is reduced by the organic mediator that is involved in the enzymatic reactions. However, activated sludge is a very complex structure created by numerous, undefined microorganisms, which makes it difficult to determine the exact oxidation mechanism.

Short-term effects of reduced graphene oxide on the anammox biomass activity at low temperatures

Anaerobic ammonium oxidation (anammox) is an efficient process for nitrogen removal from wastewater, but its common use is limited by its relatively high optimal temperature (30 °C). One of the major bottlenecks of the implementation of mainstream PN/A process is the low activity of the anammox bacteria at low temperature. Due to this reason over the past years, numerous researchers have attempted to overcome this limitation. Recently it was shown that the reduced graphene oxide (RGO) can accelerate the anammox bacteria activity. However all these studies were performed at high temperatures (over 30 °C). Thus, in this study, supporting the anammox process at low temperatures (10-30 °C) by the RGO was investigated for the first time. The statistical analysis confirmed that RGO significantly affects the anammox activity. The stimulation effect of RGO on the anammox bacteria activity is of particular importance at low temperatures, when drastic decrease in process activity is observed at temperatures below 15 °C. The short-term experimental results demonstrated stimulation of the anammox activity at 13 °C, up to 28% by 15 mg RGO/L, but concentrations above 40 mg RGO/L caused the process inhibition, up to 30% with 50 mg RGO/L. However, the effect of RGO probably depends on the nanomaterial dose per biomass unit and the optimal range of this value was evaluated as 20 to 45 mg RGO/g VSS (volatile suspended solids).

Evaluating the effects of metal oxide nanoparticles (TiO2, Al2O3, SiO2 and CeO2) on anammox process: Performance, microflora and sludge properties

The increasing use of engineered metal oxide nanoparticles (MONPs) in consumer products raises great concerns about their environmental impacts, but their potential impacts on anaerobic ammonium oxidation (anammox) process in wastewater treatment remain unclear. In this study, the presence of MONPs (1, 50, 200 mg L^-1) exhibited no visible effects on the nitrogen removal performance of anammox reactors, but high levels (200 mg L^-1) of SiO₂NPs, Al₂O₃NPs and CeO₂NPs had a distinct effect on shaping the anammox community. Long-term exposure of MONPs caused different responses in the relative abundance of Ca. Kuenenia, the level of functional gene HzsA and the activities of three key enzymes involved in anammox metabolism, but no significant inhibition effects on specific anammox activity were detected. Overall, the effects of MONPs on anammox community structure and sludge properties depended on their types and levels and followed the order SiO₂ > CeO₂ > Al₂O₃ > TiO₂.

Effects of short-term exposure to linear anionic surfactants (SDBS, SLS and SDS) on anammox biomass activity

This work demonstrates the feasibility of nitrogen removal from wastewater containing linear anionic surfactants, including sodium dodecyl benzene sulfonate, sodium lauryl sulfate and sodium dodecyl sulfonate, by using the anammox process.