Iron-dependent nitrate reduction by anammox consortia in continuous-flow reactors: A novel prospective scheme for autotrophic nitrogen removal

The interactive application and impacts of iron/nitrogen biogeochemical cycling in distributed ponds for non-point source pollution control in a watershed

The linkages of distributed ponds are utilized in conjunction with one another to remediate non-point source (NPS) pollution in a water-scarce basin. This study provides an overview of a state-of-the-art thorough evaluation of ponds, which offers insight into the majority of topics covered by the ongoing scientific studies, including their various functions and factors affecting their functioning on the hydrological, physicochemical, and biological processes, such as environmental climate factors and basin-specific landscape configuration parameters, as well as process parameters for design, operation and management aspects. The linkages of ponds provide a variety of sustainable services (6R functions), such as resources, restoration, reduction, reuse, recycling, and recovery. The significance of regional environmental geochemical substrates in the ponds, such as red soil, as a hotspot for microbial reaction is emphasized to demonstrate the significant contribution of the migration and transformation of Fe/N cycles to the pollution removal process. In this review, 178 original research publications were thoroughly analyzed to improve our knowledge of the iron-nitrogen cycle in wetlands. From a molecular biology standpoint, the identification of functional microbe species and genes linked to microbially driven iron-nitrogen cycle activities is delved. Reliable data and homogeneous datasets from 42 studies were collected. The correlation analysis results demonstrated Feammox rates contributed to the N loss amount (r = 0.871; p < 0.01), and they had a positive correlation with Fe(III) concentration (r = 0.965; p < 0.01). The proposal for the treatment of NPS pollution by large-scale linkages of ponds in a basin involves optimizing Fe/N microbial processes to promote iron crystallization and efficient circulation of Fe(II) and Fe(III). The co-benefits of geochemistry, biotechnology, and environmental science should be considered when managing contamination in engineering applications. The linkages framework for integrated ponds, which incorporates macro (watershed management) and micro (biogeochemical cycle mechanism) investigations, provides a systematic approach to the application of integrated ponds and sustainable water management for NPS pollution control.

Nitrate formation in anammox process: Mechanisms and operating conditions

Anaerobic ammonium oxidation (anammox) is an energy-efficient technology for wastewater nitrogen removal. However, the byproduct nitrate has hindered development and application of anammox process. Meanwhile, the knowledge of nitrate formation during anammox process is insufficient, which prohibits high nitrogen removal. This review firstly summaries and discusses valuable findings on nitrate formation, including molecular mechanism of nitrate production, microbial pathway of nitrate reduction and its net formation. Specially, influences of operating conditions on mechanisms and patterns of nitrate formation are analyzed. Then, based on nitrate formation mechanism, current strategies of nitrate removal from anammox process are reevaluated. Finally, the key knowledge gaps and further process development are presented. Overall, this review sheds light on the understanding of nitrate formation of anammox process, which would further facilitate and optimize the process design and operation for high performance nitrogen removal.

Influence of crop residue-induced Fe-DOC complexation on nitrate reduction in paddy soil

Although complexation between dissolved organic matter (DOM) and ubiquitous Fe is known to have a major influence on electron transferring ability in redoximorphic soil, it was unclear whether and how this complexation affected nitrate reduction and N2O productivity. The nitrate reduction of paddy soil in the presence of crop residues returning under flooding conditions was explored in this study. The rate of nitrate reduction in control soil was 0.0677 d-1, while it improved 1.99 times in treatment soil with Chinese milk vetch (CMV) straw returning. During a 28-day incubation period, N2O productivity decreased 0.08-0.91 ppb in CMV soil and 0.43-0.50 ppb in rice straw soil compared with control. The presence of crop residue increased DOC content and Fe (III) reduction rate, which aided in the formation of Fe (II)-DOC complexation. Meanwhile, the addition of CMV increased the content of DOC by 5.14-78.77 mg/kg and HCl extractable Fe (II) by 35.12-1221.03 mg/kg. Crop residues returning to soil increased the relative abundance of iron reductive and electroactive genera, as well as denitrifying genera with more copies of denitrification genes (Archangiaceae, Gemmatimonadaceae, and Burkholderiaceae). The synergistic effect of Fe-DOC complexation, electroactive genera, and denitrifying genera contributed to up-regulated expression of napA and narG (5.84 × 106 and 3.39 × 107 copies increased in the CMV soil compared to the control) numbers and equally accelerated reduction of nitrate to nitrite, while further nitrite reduction was primarily attributed to the abiotic reaction by Fe (II). From a bio-electrochemical point of view, this work provided new insight into the nitrate reduction of paddy soil impacted by Fe-DOC complexation.

Toxic effect of copper ions on anammox in IFFAS process filled with ZVI-10 modified carriers

The inhibitory effects of heavy metals on anammox bacteria (AnAOB) have attracted attention worldwide. However, most are conducted in activated sludge rather than biofilm systems. The toxic effect and resistance response of anammox biofilm are not predictable from those of free-living AnAOB. Zero valent iron (ZVI) has been demonstrated to enhance anammox performance, but whether ZVI can promote AnAOB resistance to heavy metal stress remains unclear. Herein, the toxic effect of copper ions (Cu(II)) on anammox in integrated floating-film activated sludge (IFFAS) process filled with 10 wt% ZVI modified carriers (R1) was investigated. Results indicated half inhibiting concentration (IC50) of Cu(II) in R1 was 9.13 mg/L, which was much higher than that in R0 filled with conventional carriers made of high density polyethylene (HDPE) (3.94 mg/L). Long-term effect of Cu(II) demonstrated that Cu(II) concentrations less than 1.0 mg/L could not inhibit anammox biofilm significantly, whereas R1 performed better anammox process than R0 under the stress of 0.1-1.0 mg/L Cu(II). The ZVI modified carriers induced more extracellular polymeric substances (EPS) to trap Cu(II) to attenuate the toxicity to AnAOB. Besides, the activities of functional enzymes related to anammox (NIR and HDH), as well as heme-c contents, were always higher in R1 than R0 regardless of the Cu(II) dosage. Candidatus Kuenenia was identified as the predominant AnAOB, which had stronger resistance to Cu(II) stress compared to other genera in the IFFAS process. Metal resistance genes (MRGs) analysis identified AnAOB induced multi-responses to resist Cu(II) stress, such as the up-regulation of copC, cutA, cutC, cutF, cueR and cueO, to synthesize more proteins with functions of copper exocytosis, conjugation and oxidation.

Enhancement of nitrogen removal in coupling Anammox and DAMO via Fe-modified granular activated carbon

Anaerobic ammonium oxidation (Anammox) coupled with Denitrifying anaerobic methane oxidation (DAMO) is an attractive technology to simultaneously remove nitrogen and mitigate methane emissions from wastewater. However, its nitrogen removal rate is usually limited due to the low methane mass transfer efficiency, low metabolic activity and slow growth rate of functional microorganisms. In this study, GAC and Fe-modified GAC (Fe-GAC) were added into Anammox-DAMO process to investigate their effects on nitrogen removal rates and then reveal the mechanism. The results showed that after 80-day experiments, the total nitrogen removal rate was slightly improved in the presence of GAC (3.94 mg L^-1·d^-1), while it reached high as 16.66 mg L^-1·d^-1 in the presence of Fe-GAC, which was ca.17 times that of non-amended control group (0.96 mg L^-1·d^-1). The addition of Fe-GAC stimulated the secretion of extracellular polymeric substance (EPS), improved the electron transfer capability and promoted the production of Cytochrome C. Besides, the key functional enzyme activities (HZS, HDH and NAR) were highest in the Fe-GAC group, which were approximately 1.06-1.56 times higher than those of GAC-amended and blank control groups. Microbial community analysis showed that the abundance of the DAMO archaea (Candidatus Methanoperedens) and Anammox bacteria (Candidatus Brocadia) were remarkably increased with the addition of Fe-GAC. Functional genes associated with nitrogen removal and methane oxidation in Fe-GAC system were up-regulated. This study provides a promising strategy for achieving high rate of nitrogen removal upon Anammox-DAMO process.

Performance, comparison and utilization of reduced sulfur (-2) compounds (S2-, FeS and SCN-) in autotrophic denitrification process by thiosulfate-driven autotrophic denitrifier

The coexistence of reduced sulfur (-2) compounds (S2-, FeS and SCN-) are found in some industrial wastewaters due to pre-treatment of Fe(II) salts. These compounds as electron donors have attracted increasing interest in autotrophic denitrification process. However, the difference of their functions still remain unknown, which limit efficient utilization in autotrophic denitrification process. The study aimed to investigate and compare utilization behavior of these reduced sulfur (-2) compounds in autotrophic denitrification process activated by thiosulfate-driven autotrophic denitrifiers (TAD). Results showed that the best denitrification performance was observed in SCN-; while the reduction of nitrate was significantly inhibited in S2- system and the efficient accumulation of nitrite was observed in FeS system with cycle experiments continuing. Additionally, intermediates containing sulfur were produced rarely in SCN- system. However, the utilization of SCN- was limited obviously in comparison with S2- in coexistence systems. Moreover, the presence of S2- increased the accumulation peak of nitrite in coexistence systems. The biological results indicated that the TAD utilized rapidly these sulfur (-2) compounds, in which genus of Thiobacillus, Magnetospirillum and Azoarcus might play main roles. Moreover, Cupriavidus might also participate in sulfur oxidation in SCN- system. In conclusion, these might be attributed to the characteristics of sulfur (-2) compounds including the toxicity, solubility and reaction process. These findings provide theoretical basis for regulation and utilization of these reduced sulfur (-2) compounds in autotrophic denitrification process.

Mitigating mechanism of nZVI-C on the inhibition of anammox consortia under long-term tetracycline hydrochloride stress: Extracellular polymeric substance properties and microbial community evolution

In this study, activated carbon-loaded nano-zero-valent iron (nZVI-C) composites were added to anaerobic ammonium oxidation bacteria (AnAOB) to overcome the inhibition of tetracycline hydrochloride (TCH). Results showed that 500 mg L^-1 nZVI-C effectively mitigated the long-term inhibition of 1.5 mg L^-1 TCH on AnAOB and significantly improved the total nitrogen removal efficiency (TNRE) (from 65.27% to 86.99%). Spectroscopic analysis revealed that nZVI-C increased the content of N-H and CO groups in EPS, which contributed to the adsorption of TCH. The accumulation of humic acid-like substances in EPS was also conducive to strengthening the extracellular defense level. In addition, TCH-degrading bacteria (Clostridium and Mycobacterium) were enriched in situ, and the abundance of Ca. Brocadia was significantly increased (from 10.69% to 18.59%). Furthermore, nZVI-C increased the abundance of genes encoding tetracycline inactivation (tetX), promoted mineralization of TCH by 90%, weakening the inhibition of TCH on microbial nitrogen metabolism. nZVI-C accelerated the electron consumption of anammox bacteria by upregulating the abundance of electron generation genes (nxrA, hdh) and providing electrons directly.

Nitrogen removal in improved subsurface wastewater infiltration system: Mechanism, microbial indicators and the limitation of phosphorus

To enhance the nitrogen removal capacity, scrap iron filings and Si-Al porous clay mineral material (PCMW) was used to improve a subsurface wastewater infiltration system (SWIS). The results showed TN and NH₄⁺-N removal efficiencies of improved SWIS were 20.72% and 5.49% higher than those of the control SWIS, respectively. Based on the response of the removal performance, microbial community and function analysis of 16s rRNA amplicon sequencing results, the amending soil matrix substantially enriched the nitrogen removal bacteria (Rhizobiales_Incertae_Sedis and Gemmatimonadaceae), and significantly improved the activities of key enzymes (Hao, NasAB, NarGHI, NirK, NorBC, NirA and NirBD), particularly at co-occurrence zone of nitrification and denitrification (70-130 cm depth). The amending soil matrix not only extended the growth space of microbes, but also provided additional electrons and carbon sources for denitrifying bacteria by regulating the structure and function of the microbial community. In addition, amending soil matrix could enhance phosphate metabolism genes and phosphate solubilizing microbes in the denitrification zone by increasing the phosphorus source, thus strengthening nitrogen metabolism. Nitrospiraceae, Rhizobiales_Incertae_Sedis and Gemmatimonadaceae related to nitrogen removal and Bacillaceae with phosphate-solubilizing ability could be used as microbial indicators of nitrogen removal in SWISs. The reciprocal action of environmental on microbial characteristics exhibited microbial functional were related to DO, Fe²⁺, TOC, TP, TN, NH₄⁺-N and NO₃^--N. Those could be used as physicochemical and biological indicators for application and monitoring of SWIS. In conclusion, this study provided a low-cost and efficient enhancement approach for the application of SWIS in decentralized domestic sewage treatment, and furnished theoretical support for subsequent applications.

Treatment of micro-polluted water with low C/N ratio by immobilized bioreactor using PVA/sintered ores@sponge cube: Performance effects and potential removal pathways

The widespread use of industrial products containing lead (Pb²⁺) and tetracycline (TC) medications led to the combined pollution of nitrate, Pb²⁺, and TC in water. A novel biomaterial containing polyvinyl alcohol (PVA) and sponge cube with sintered ores (PVA/sintered ores@sponge cube) was prepared to ensure the maximum NO₃^--N removal efficiency (96.21 %) of the bioreactor under the hydraulic retention time (HRT) of 7.0 h, pH of 6.0, and the carbon to nitrogen (C/N) of 1.5 that had the ability to remove TC and Pb²⁺ synergistically. Composite pollutants slightly decreased denitrification performance in the combined pollution system on account of the addition of sintered ores. Results of scanning electron microscopy (SEM) showed that the sintered ores in the biocarrier induced denitrification and the adsorption of bio‑iron oxides were involved in the removal of TC and Pb²⁺. The simultaneous removal of composite pollutants during denitrification was facilitated by extracellular polymeric substances (EPS) as revealed by Fourier transform infrared spectroscopy (FTIR) and fluorescence excitation-emission matrix (EEM). In addition, high-throughput sequencing results showed that Zoogloea had the highest proportion in the bioreactor.

Strategy of nitrate removal in anaerobic ammonia oxidation-dependent processes

The anaerobic ammonium oxidation (anammox), a microbial process that is considered as a low-cost and high efficient wastewater treatment, has received extensive attention with an attractive application prospect. The anammox process reduces nitrite (NO₂^-) to nitrogen gas (N₂) with ammonium (NH₄⁺) as the electron donor. However, some nitrate (NO₃^-) equivalent to 11% of total nitrogen (TN) is generated in this process, which limits the development of anammox. To overcome this problem, many efforts have been made in this regard, mainly combining with other biological treatment methods (denitrification, denitrifying anaerobic methane oxidation, etc.), introducing the substance into anammox process, etc. Herein, we summarized a detailed review of previous researches on the removal of NO₃^- in the anammox-dependent processes. It is hoped that this review could serve as valuable guidance in future research and practical applications of anammox.

Discovering dominant ammonia assimilation: Implication for high-strength nitrogen removal in full scale biological treatment of landfill leachate

Landfill leachate containing high-strength nitrogen is generated in domestic waste landfilling. The integration of anoxic and aerobic process (AO) based on nitrification and denitrification, has been a mainstream process of biological nitrogen removal (BNR). But the high-strength organics as well as aerobic effluent reflux might change the biochemical environment designed and operated as AO. In view of the nitrogen balance in a full scale landfill leachate treatment plant with two-stage AO, we found that approximately 90% removal of total nitrogen (TN) and ammonia (NH₄⁺-N) focused on primary anoxic and aerobic stage. Meanwhile, the less nitrate and nitrite in the aerobic effluent were incapable of sustaining denitrification or anaerobic ammonia oxidation (anammox). The high reflux flow from aerobic to anoxic process enabled the similar microbial community and functional genes in anoxic and aerobic process units. However, the functional genes involving ammonia assimilation in all process units showcased the highest abundance compared to those correlated with other BNR pathways, including nitrification and denitrification, assimilatory and dissimilatory nitrate reduction, nitrogen fixation and anammox. The ammonia assimilation dominated the removals of TN and NH₄⁺-N, rather than other BNR mechanism. The insight of dominant ammonia assimilation is favorable for illustrating the authentic BNR mechanism of landfill leachate in AO, thereby guiding the optimization of engineering design and operation.

Activity enhancement and the anammox mechanism under low temperature via PVA-SA and nano Fe2O3-PVA-SA entrapped beads

Anaerobic ammonia-oxidizing bacteria (AAOB) have a long growth time and low activity at low temperatures. In suspended systems, sludge is easily lost, which limits the mainstream application of anaerobic ammonia oxidation (anammox).Entrapment provides effective ideas for solving these problems. In this study, polyvinyl‑sodium alginate (PVA-SA) and nano Fe₂O₃-PVA-SA entrapment beads were prepared to discuss the effectiveness of entrapment enhanced anammox sludge at low temperatures. The differences in the entrapped beads and granules were compared to analyze the strengthening mechanism. The results show that the nitrogen removal performance of granules, PVA-SA and nano Fe₂O₃-PVA-SA entrapped beads, first decreased and then increased during the cooling and low-temperature operation. Nano Fe₂O₃-PVA-SA entrapped beads showed the smallest decline and the highest degree of recovery. Reaction metering ratio (△NO₂^--N/△NH₄⁺-N and △NO₃^--N/△NH₄⁺-N) showed that entrapment could realize Nitrite oxidizing bacteria (NOB) inhibition and improve the activity of denitrifying bacteria (DNB) to promote the removal of total nitrogen by providing a strict anaerobic environment. The results demonstrate that entrapment is beneficial for maintaining the content of heme c, specifically, nano Fe₂O₃ can stimulate its production, and is beneficial for alleviating the reduction of hydrazine dehydrogenase (HDH) enzyme activity. The extracellular polymeric substances (EPS) content and analysis showed that entrapment does not change the composition of EPS, and can maintain the EPS content. Nano Fe₂O₃ can stimulate AAOB to secrete more EPS to maintain sludge stability. From a molecular perspective, entrapment can maintain the expression of functional genes, promote the enrichment of AAOB, thus improving the nitrogen removal performance from the dual perspectives of "quality" and "quantity".

Microbial driving mechanism for simultaneous removal of nitrogen and phosphorus in a pure anammox reactor under ferrous ion exposure

The mechanisms of Fe²⁺ on nitrogen and phosphorus removal and functional bacterial competition in anammox systems was investigated. Under 0.12 mM Fe²⁺, the performance of nitrogen and phosphorus removal increased by 10.08 % and 151.91 %, respectively, compared with the control stage. Phosphorus removal was achieved through extracellular polymeric substance (EPS) induced biomineralization to form Fe-P minerals, and functional group COC in EPS played a critical role. T-EPSs was the major nucleation site due to it maintaining the supersaturated state (saturation index > 0) of Fe-P minerals for a long time. Population succession showed that Fe²⁺ weakened the competition between heterotrophic denitrifier (Denitrasoma) and anammox microbe (Candidatus Brocadia) for space and substrates, which was favorable for the enrichment of anammox biomass. Moreover, the variation in gene abundance (such as Hao, Cyt c, and Nir) indicated that Fe²⁺ improved electron behaviors (generation, transport, and consumption) during the nitrogen metabolism of anammox systems.

Synthesis of self-renewing Fe(0)-dispersed ordered mesoporous carbon for electrocatalytic reduction of nitrates to nitrogen

In electrocatalytic reduction of nitrates to nitrogen, key issues are electrode activity, sustainable materials, preparation methods and cost. Herein, lignin, Fe³⁺ ion, and non-ionic surfactant were combined with evaporation-induced self-assembly (EISA) to prepare zero-valent Fe-dispersed ordered mesoporous carbon (OMC) electrode materials denoted as Fe#OMC. The method developed for preparing Fe-coordinated OMC material avoids the use of toxic phenols, aldehyde reagents and metal doping compounds. When synthesized Fe#OMC samples were applied as electrode materials for the electrocatalytic reduction of nitrate in aqueous solutions, maximum nitrate nitrogen removal was as high as 5373 mg N·g^-1 Fe from aqueous solutions containing 400 mg·L^-1 NO₃^--N, while nitrogen selectivity was close to 100%, exceeding catalytic performance of comparable materials. Active hydrogen produced by electrolysis of water during the reaction re-reduced Fe ions formed in the OMC material and stabilized Fe#OMC electrode performance and recycle. The Fe#OMC electrode is self-renewing with respect to its Fe zero-valent state, is simple to prepare from sustainable materials and is effective for electrocatalytic reduction of nitrate or nitrogen-containing compounds in water.

Efficient nitrogen removal from leachate by coupling Anammox and sulfur-siderite-driven denitrification

High concentrations of nitrate can be generated during anaerobic ammonium oxidation (Anammox) wastewater treatment processes. Addition of sulfur to Anammox reactors stimulates the growth of sulfur-driven denitrifying (SADN) bacteria that can reduce nitrate to nitrogen gas. However, protons released during the SADN process lower the pH of the system and inhibit Anammox activity. The system will keep stable when pH is in the range of 7.5-8.5. This study showed that addition of siderite stabilized the reactor system and significantly improved the nitrogen removal process. In fact, even when concentrations of total nitrogen were 477.15 ± 16.84 mg/L, the sulfur/siderite reactor maintained nitrogen removal efficiencies >90%, while efficiencies in the sulfur reactor were < 80%. Anammox accounted for 31% of the bacterial sequences in the sulfur/siderite reactor compared to only 14% in the sulfur reactor with the majority of sequences clustering with Ca. Brocadia. An abundance of c-type cytochromes in anammox aggregates in the sulfur-siderite reactor also indicated that anammox activity was higher in this system.

Metagenomics illuminated the mechanism of enhanced nitrogen removal and vivianite recovery induced by zero-valent iron in partial-denitrification/anammox process

In this study, a novel strategy of zero-valent iron (ZVI) combined with acetic acid was proposed to optimize partial-denitrification/anammox (PD/A) process, and enhanced nitrogen removal mechanism was elucidated through metagenomics. Results showed that the optimal nitrogen and phosphorus removal were as high as 99.50% and 98.37%, respectively, with vivianite being precipitated as the main byproduct. The occurrence of Feammox was a crucial link for enhanced ammonia removal and vivianite recovery. Metagenomic analysis further certified that long-term acclimation of optimization strategy triggered DNRA-based nitrate reducing genes (narY/Z and nrfA) assigned to Candidatus Brocadia, which allow direct uptake of nitrate by the anammox. Additionally, ZVI might act as a new electron donor to decrease organics dependence of PD by reducing the abundance of genes for electron production involved in carbon metabolism. However, FA addition enhanced the relative abundances of genes involved in anammox including nitrogen reduction and oxidation, thereby accelerating nitrogen removal.

Full-Scale Application of One-Stage Simultaneous Nitrification and Denitrification Coupled with Anammox Process for Treating Collagen Casing Wastewater

The ammonia nitrogen (NH₄⁺-N) concentration in the effluent released from the secondary sedimentation tank of the original collagen enteric coating wastewater treatment process considerably exceeded the Chinese effluent discharge standard. Therefore, a one-stage simultaneous nitrification and denitrification coupled with the anaerobic ammonia oxidation (SNDA) process was designed to terminally treat collagen enteric coating wastewater containing low COD/NH₄⁺-N (C/N). The entire process start-up and NH₄⁺-N loading (NLR) domestication phase was completed within two months. During the NLR domestication, the NH₄⁺-N removal rate was more than 90% and its effluent concentration was less than 15 mg/L, guaranteeing that the NH₄⁺-N in the subsequent effluent was within the standard value. The results of microbial diversity show that Acinetobacter, Bacillus, and other heterotrophic nitrification–aerobic denitrification bacteria, and anammox ammonia oxidation bacteria were the main functional bacteria at the genus level, exhibiting high denitrification performance. The one-stage SNDA process effectively and stably removed nitrogen; the treated sewage satisfied the national comprehensive wastewater discharge standard (GB8978-1996), effectively saving 30–40% of the floor area and reducing 67.6% of the additionally added alkali, wherein the system’s denitrifying bacteria compensated for some alkali consumed during the nitrification reaction.

Impact mechanism and performance enhancement of ultrasound on ZVI-anammox system

The zero-valent iron-anaerobic ammonium oxidation (ZVI-anammox) system has received widespread attention due to its excellent nitrogen removal performance and user-friendly operation. However, its disadvantages include a short service life, high ZVI consumption, and poor system stability. The use of ultrasound as a physical method is increasing in various water treatment processes. In this study, a series of batch tests were conducted to obtain the best ultrasonic parameter and explore the comprehensive effects of ultrasound on a ZVI-anammox system. The highest specific anammox activity of the ZVI-anammox system was found to be 2.88 mg total nitrogen/g of volatile suspended solids/h after ultrasonic treatment (0.2 w/mL, 5 min), which was 37.85% higher than a control group. Additionally, the service life of ZVI extended by 28.57% and the total nitrogen removal efficiency changed from 58.03-72.08 to 63.92-78.33% under ultrasonic irradiation. These phenomena were related to the mechanical force and cavitation of ultrasonic waves. Judging from the characteristics of sludge and ZVI, ultrasound can promote anammox sludge granulation, ease ZVI passivation, and enhance the stability of the entire system. This paper also briefly discusses the impact mechanisms of ultrasound on the ZVI-anammox system. In brief, ultrasound destroys the surface structure of ZVI and thus provides numerous attachment points for microorganisms that improve the performance of the entire system. The proposed ultrasound combined with ZVI is a novel method that has potential for use in large-scale engineering applications in actual sewage treatment after comprehensive analysis.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Anammox-synchronous zero-valent iron oxidation promoting synergistic nitrogen and phosphorus removal from wastewater

In this study, a novel process, anammox-synchronous zero-valent iron (ZVI) oxidationin whichnitrate byproductsare usedfor in situoxidization of ZVIwas first developed to simultaneously remove nitrogen and phosphorus from wastewater. The optimal ZVI dosage of 4 g/L significantly improved the nitrogen removal efficiency to 86.02 ± 1.98%, with the highest phosphorus removal efficiency enhanced from 39.62% to 98.97%. Several analytical techniques revealed that iron phosphate minerals formed by biologically induced mineralization promoted the phosphorus removal of the system and enhanced the settleability of granules. Candidatus Brocadia was the main anammox functional bacteria, which could promote the formation of iron phosphorus minerals. The increase of key functional genes involved in denitrification, especially narG, played a pivotal role in reducing nitrate to improve nitrogen removal performance. In addition, abundance regulation of gene fur enabled anammox bacteria always maintain high activity under different pH and ZVI dosages.

Insight into the structure and metabolic function of iron-rich nanoparticles in anammox bacteria

Anaerobic ammonium-oxidizing (anammox) bacteria are iron abundant and depend heavily on iron-binding proteins. The iron demand of anammox bacteria is relatively large. However, it still remains some doubts where these large quantities of available iron come from and how they are regulated in anammox bacteria. Herein, iron-rich nanoparticles in anammoxosomes were detected by synchrotron soft X-ray tomography coupled with scanning transmission X-ray microscopy (STXM). The iron-rich nanoparticles were identified as ferric oxide (α-Fe₂O₃) mineral cores, and the local atomic structure of iron-rich nanoparticles was obtained by X-ray absorption fine-structure (XAFS) spectra. The bacterioferritin of Q1Q315 and Q1Q5F8 were detected by proteomics analysis. On this basis, the metabolic pathway centered on iron-rich nanoparticles was proposed.

A mechanistic model for denitrifying anaerobic methane oxidation coupled to dissimilatory nitrate reduction to ammonium

Nitrate/nitrite-dependent anaerobic methane oxidation (n-DAMO) is an important process linking nitrogen and carbon cycle. It is recently demonstrated that n-DAMO archaea are able to couple n-DAMO to dissimilatory nitrate reduction to ammonium (DNRA). In this work, a mathematical model is developed to describe DNRA by n-DAMO archaea for the first time. The anabolic and catabolic processes of n-DAMO archaea, n-DAMO bacteria and anaerobic ammonium oxidation (Anammox) bacteria are involved. The different impacts of exogenous and endogenous nitrite on DNRA and n-DAMO microbes are considered. The developed model is calibrated and validated using experimental data collected from a sequencing batch reactor (SBR) and a counter-diffusion membrane biofilm bioreactor (MBfR). The model outputs fit well with the profiles of nitrogen (N) dynamics and biomass changes in both reactors, demonstrating its good predictive ability. The developed model is further used to simulate the counter-diffusion MBfR incorporating n-DAMO and Anammox process to treat sidestream wastewater. The simulated distribution profiles of N removal/production rates by different microbes along biofilm depth reveal that DNRA by n-DAMO archaea plays an important role in N transformation of the integrated n-DAMO and Anammox process. It is further suggested that the counter-diffusion MBfR under the investigated conditions should be operated at proper hydraulic retention times (HRTs) (i.e. 6h and 8h) with exogenous NO₂^- in the range of 0-10 mg N/L or at HRTs >3h with the absence of exogenous NO₂^- in order to achieve dischargeable effluent.

Green approach and strategies for wastewater treatment using bioelectrochemical systems: A critical review of fundamental concepts, applications, mechanism, and future trends

Millions of litters of multifarious wastewater are directly disposed into the environment annually to reduce the processing costs leading to eutrophication and destroying the clean water sources. The bioelectrochemical systems (BESs) have recently received significant attention from researchers due to their ability to convert waste into energy and their high efficiency of wastewater treatment. However, most of the performed researches of the BESs have focused on energy generation, which created a literature gap on the utilization of BESs for wastewater treatment. The review highlights this gap from various aspects, including the BESs trends, fundamentals, applications, and mechanisms. A different review approach has followed in the present work using a bibliometric review (BR) which defined the literature gap of BESs publications in the degradation process section and linked the systematic review (SR) with it to prove and review the finding systematically. The degradation mechanisms of the BESs have been illustrated comprehensively in the current work, and various suggestions have been provided for supporting future studies and cooperation.

Novel biotechnologies for nitrogen removal and their coupling with gas emissions abatement in wastewater treatment facilities

Wastewaters contaminated with nitrogenous pollutants, derived from anthropogenic activities, have exacerbated our ecosystems sparking environmental problems, such as eutrophication and acidification of water reservoirs, emission of greenhouse gases, death of aquatic organisms, among others. Wastewater treatment facilities (WWTF) combining nitrification and denitrification, and lately partial nitrification coupled to anaerobic ammonium oxidation (anammox), have traditionally been applied for the removal of nitrogen from wastewaters. The present work provides a comprehensive review of the recent biotechnologies developed in which nitrogen-removing processes are relevant for the treatment of both wastewaters and gas emissions. These novel processes include the anammox process with alternative electron acceptors, such as sulfate (sulfammox), ferric iron (feammox), and anodes in microbial electrolysis cells (anodic anammox). New technologies that couple nitrate/nitrite reduction with the oxidation of methane, H₂S, volatile methyl siloxanes, and other volatile organic compounds are also described. The potential of these processes for (i) minimizing greenhouse gas emissions from WWTF, (ii) biogas purification, and (iii) air pollution control is critically discussed considering the factors that might trigger N₂O release during nitrate/nitrite reduction. Moreover, this review provides a discussion on the main challenges to tackle towards the consolidation of these novel biotechnologies.

Coupling oxidation of acid volatile sulfide, ferrous iron, and ammonia nitrogen from black-odorous sediment via autotrophic denitrification-anammox by nitrate addition

The coupling removal of acid volatile sulfide (AVS), ferrous iron, and ammonia nitrogen has been applied for black-odorous sediment remediation. In this study, calcium nitrate with different N/(S + Fe) ratios (0.45, 0.90, 1.20 and 1.80) was added into black-odorous sediment in four systems named R1, R2, R3, and R4. Results showed that the removal rate of AVS was 76.40% in the R1, which was lower compared with rates in R2-R4 around 96.70%. The ferrous oxidation rate was approximately 87.00% in R2-R4, which was considerably higher than that in the R1 (24.62%). And the ammonia was reduced by 81.02%, 88.00%, 100%, and 57.18% in R1, R2, R3 and R4, respectively. During the reaction, nitrite accumulation was observed, indicating partial denitrification. Moreover, microbes related to autotrophic denitrification (e.g., genus Thiobacillus, Dok59, GOUTA19, Gallionella, with the highest abundance of 15.40%, 13.21%, 8.79%, 9.44%, respectively) were detected in all systems. Furthermore, the anammox bacteria Candidatus_Brocadia with the highest abundance of 3.44% and 4.00% in R2 and R3, respectively was also found. These findings confirmed that AVS, ferrous iron, and ammonia nitrogen could be simultaneously removed via autotrophic denitrification coupled with anammox in black-odorous sediment by nitrate addition.

Nitrate removal by anammox biomass with intracellular carbon source as electron donors via DNRA pathway

In this work, a novel nitrate (NO₃^-) reduction pathway by anaerobic ammonium oxidation (anammox) biomass was firstly discovered with the intracellular carbon sources as the only electron donors. And the possible reaction mechanism was deduced to be intracellular dissimilatory nitrate reduction to ammonium (DNRA) pathway according to the experimental results. In batch experiments, without any external electron donors, NO₃^--N (about 50 mg/L) was reduced to N₂ within 48 h, and a small amount of NO₂^--N was detected (the maximum of 2 mg/L) with the anammox biomass concentration of 4400 mg/L. Acetylene (4.46 mmol/L) addition resulted in obvious NH₄⁺ accumulation during NO₃^- degradation by anammox biomass, since acetylene mainly interfered in hydrazine (N₂H₄) generation from NH₄⁺ and NO. Without HCO₃^- addition, the NO₃^--N degradation rate was slower than that with HCO₃^- addition. Simultaneously, glycogen contents inside anammox biomass decreased to 133.22 ± 1.21 mg/g VSS and 129.79 ± 1.21 mg/g VSS with and without HCO₃^-, respectively, from 142.20 ± 0.61 mg/g VSS. In the long-term experiment, anammox biomass stably degraded NO₃^--N without external electron donors addition, and the maximum removal efficiency of NO₃^--N reached 55.4%. The above results indicated the anammox bacteria utilized the DNRA pathway to reduce NO₃^- to NO₂^- and further NH₄⁺, then normal anammox metabolism would continue to convert the produced NO₂^- and NH₄⁺ to N₂. The intracellular stored carbon sources (e.g., glycogen) were supposed to be electron donors for NO₃^- degradation. This capability would enhance the viability and living space of anammox bacteria in different natural ecosystems, and make it plausible that complete nitrogen removal could be implemented only by the anammox process.

Simultaneous nitrate, nickel ions and phosphorus removal in a bioreactor containing a novel composite material

This study presents the novel composite material TMCC/PAA/SA@Fe(TPSA), a bacteria immobilized carrier for use in bioreactor systems to enhance the simultaneous removal efficiency of nitrate, Ni(II) and phosphorus. The influence of various operational factors were evaluated on the performance of nitrate, phosphorus and Ni(II) removal. Results demonstrate that under optimum conditions of an hydraulic retention time (HRT) of 8 h and pH 7.0, nitrate and phosphorus removal reached nearly 100% and 61.7%, respectively. When the initial Ni(II) concentration was 1 mg/L, approximately 100% Ni(II) removal efficiency was achieved. Furthermore, the morphology and components of the TPSA immobilized bacterial pellets were analyzed to investigate the mechanism of simultaneous nitrate, Ni(II) and phosphorus removal. Microbial metabolism was more active in the experimental reactor compared with control, although high concentrations of Ni(II) could inhibit bacterial activity.

Impacts of Chosen Parameters on Fe-Dependent Nitrate Reduction in Anammox Consortia: Performance and Bioactivity

Fe-dependent nitrate reduction by anammox consortia could serve as a valuable autotrophic denitrification process for wastewater treatment. However, influences of temperature, pH, and Fe/NO3-N ratio on this biochemical process have not been studied. The present study investigated individual and interactive effects of aforementioned parameters on nitrate removal performance and bioactivity of anammox consortia via a series of batch assays. Enzymes activity of nitrate reductase (NAR) and hydrazine dehydrogenase (HDH) of anammox consortia had high consistency with nitrogen removal performance and significantly depended on temperature and Fe/NO3-N ratio, while the narG and hdh genes expression were drastically depressed by extreme temperature. Models developed by response surface methodology (RSM) showed the significance of individual parameter followed by Fe/NO3-N ratio > temperature > pH, while combined effects of temperature versus Fe/NO3-N ratio exerted the most significant impacts. The pH in range of 4.0–8.0 had less influence. The optimum condition for nitrate removal efficiency (NRE) > 90% and total nitrogen removal efficiency (TNRE) > 75% was 4.0–7.4 for pH, 25.5–30.0 °C for temperature, and 31–48 for Fe/NO3-N molar ratio. The maximum NRE and TNRE could be 98.68% and 79.42%, respectively, under the condition of pH = 4.00, temperature = 28.5 °C and Fe/NO3-N ratio = 37.4. The models showed good dependability for simulation nitrogen removal performance by anammox in the real semiconductor wastewater.