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

High-level nitrogen removal achieved by Feammox-based autotrophic nitrogen conversion

Anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox) is an essential process in the geochemical iron and nitrogen cycling. This study explores Feammox-based nitrogen removal in a continuous laboratory up-flow bioreactor stimulated by intermittently adding 5 mM Fe(OH)3 at intervals of approximately two months. The feed was synthetic wastewater with a relatively low ammonium concentration (∼100 mg N/L), yet without organic carbon in order to test its autotrophic nitrogen removal performance. The operation of this system showed the achievement of high-level ammonium and total nitrogen removal efficiency (∼97% and ∼90% on average, respectively) within four months of operation, along with a relatively practical rate of ∼50 mg N/(L·d). The demand of Fe(Ⅲ) for ammonium removal during the whole bioreactor operation was estimated to be only 0.033, two orders of magnitude less than that calculated based on the Feammox reaction producing nitrogen gas. A series of assays on Fe(II) oxidation with different oxidants (O2, NO2 - and NO3 -) in abiotic and biotic batch tests further revealed an important role of Fe(II) oxidation processes, likely driven by microbial nitrate reduction and chemical oxygen reduction, in assisting the regeneration of Fe(III) for continuous Feammox-based nitrogen removal. This work demonstrates that Feammox-based autotrophic nitrogen conversion is a potential option for future wastewater treatment.

Effects of iron-carbon on nitrogen metabolism of floc and aerobic granular sludge

The aerobic granular sludge (AGS) process had been extensively studied for its simultaneous nitrification and denitrification (SND) capabilities. Iron-carbon (IC) had enhanced AGS nitrogen removal efficiency, but the mechanism remained unclear. In this study, four reactors had been added with 50, 30, 10, and 0 g/L of IC. Total nitrogen removal efficiency increased with IC dosage under the same operation mode. IC enhanced sludge ammonia oxidation rate, denitrification rate, and specific oxygen uptake rate, allowing SND to complete 60 min earlier, potentially reducing wastewater treatment costs. Notably, IC eliminated nitrite accumulation in conventional AGS effluent. IC decreased the abundance of genes and enzyme activities related to NOR expression, while increasing those related to NOS, which may mitigate the potential for nitrous oxide formation by microorganisms. In this study, IC acted as an enzymatic reaction activator, affecting granules more than flocs, with the activity gap gradually decreasing with the IC dosage.

Novel insights into Feammox coupled with the NDFO: A critical review

Ammonium oxidation coupled with Fe(III) reduction, known as Feammox, and nitrate-dependent ferrous oxidation (NDFO) are two processes that can be synergistically achieved through the Fe(III)/Fe(II) cycle. This integrated approach enables the simultaneous removal of ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) from wastewater, representing a novel method for complete nitrogen removal. This study presents a systematic and exhaustive examination of the Feammox-NDFO coupled process. An initial thorough exploration of the underlying mechanisms behind the coupling process is conducted, highlighting how the Fe(III)/Fe(II) cycle enables the concurrent occurrence of these reactions. Further, the functional microorganisms associated with and playing a crucial role in the Feammox-NDFO process are summarized. Next, the key influencing factors that govern the efficiency of the Feammox-NDFO process are explored. These include parameters such as pH, temperature, carbon source, iron source, nitrogen source, and various electron shuttles that may mediate electron transfer. Understanding the impact of these factors is essential for optimizing the process. The most recent trends and endeavors on the Feammox-NDFO coupling technology in wastewater treatment applications are also examined. This includes examining both laboratory-scale studies and field trials, highlighting their successes and challenges. Finally, an outlook is presented regarding the future advancement of the Feammox-NDFO technology. Areas of improvement and novel strategies that could further enhance the efficiency of simultaneous nitrogen removal from the iron cycle are discussed. In summary, this study aspires to offer a thorough comprehension of the Feammox-NDFO coupled process, with a focus on its mechanisms, influencing factors, applications, and prospects. It is anticipated to yield invaluable insights for the advancement of process optimization, thus sparking fresh ideas and strategies aimed at accomplishing the thorough elimination of nitrogen from wastewater via the iron cycle.

Partial nitrification-denitrification and enrichment of paracoccus induced by iron-chitosan beads addition in an intermittently-aerated activated sludge system

Insufficient carbon source has become the main limiting factor for efficient nitrogen removal in wastewater treatment. In this study, an intermittently-aerated activated sludge system with iron-chitosan (Fe-CS) beads addition was proposed for nitrogen removal from low C/N wastewater. By adding Fe-CS beads, partial nitrification-denitrification (PND) process and significant enrichment of Paracoccus (with ability of iron reduction/ammonium oxidation/aerobic denitrification) were observed in the reactor. The accumulation rate of NO2--N reached 81.9 %, and the total nitrogen removal efficiency was improved to 93.9 % by shortening the aeration time. The higher activity of ammonium oxidizing bacteria and inhibited activity of nitrite-oxidizing bacteria in Fe-CS assisted system mediated the occurrence of PND. In contrast, the traditional nitrification and denitrification process occurred in the control group. The high-throughput sequencing analysis and metagenomic results confirmed that the addition of Fe-CS induced 77.8 % and 54.9 % enrichment of Paracoccus in sludge and Fe-CS beads, respectively, while almost no enrichment was observed in control group. Furthermore, with the addition of Fe-CS beads, the expression of genes related to outer membrane porin, cytochrome c, and TCA was strengthened, thereby enhancing the electron transport of Fe(Ⅱ) (electron donor) and Fe(Ⅲ) (electron acceptor) with pollutants in the periplasm. This study provides new insights into the direct enrichment of iron-reducing bacteria and its PND performance induced by the Fe-CS bead addition. It therefore offers an appealing strategy for low C/N wastewater treatment.

Highly selective electrochemical reduction of nitrate via CoO/Ir-nickel foam cathode to treat wastewater with a low C/N ratio

Thorough nitrate removal from reclaimed water by biological techniques without carbon sources is difficult. Flexible, controllable electrochemical nitrate reduction is widely researched. Herein, ultrathin CoO nanosheets were constructed through amino group induction and orientation. The interfacial electron transfer resistance of two-dimensional CoO was 43.4% lower than that of one-dimensional nanoparticles, resulting in higher current density and improved nitrate reduction efficiency. Nickel foam and IrO2-nickel foam electrodes have almost no effect on nitrate reduction. It is worth noting that iridium loading on CoO (nanosheet) regulated the electronic band structure and generated active atomic H* . The nitrate removal rate increased from 45.1% (CoO (nanoparticle)-nickle foam) and 63.8% (CoO (nanosheet)-nickle foam) to 94.64% (CoO/Ir10 wt%-nickle foam). The proton enhancement effect improved indirect nitrate reduction by atomic H* and increased the NO3--N removal rate to 99.8%. Active chlorine species generated by Cl- in the wastewater selectively converted more than 99% of nitrate to N2, exceeding previous Co-based cathode results. In situ DEMS indicated that electrochemical reduction of nitrate included deoxidation (NO3-→*NO2-→*NO→*N/*N2O→N2) and hydrogenation (*NH2→*NH3→NH4+). The NO3--N removal rate of CoO/Ir10 wt% exceeded 65% during treatment of wastewater treatment plant effluents, verifying the feasibility of electrochemical nitrate reduction with the CoO/Ir10 wt% cathode. A strategy for designing electrochemical nitrate reduction electrocatalysts with excellent potential for full-scale application to treat wastewater treatment plant effluent is provided.

Coupling Iron Sludge Addition and Intermittent Aeration for Achieving Simultaneous Methanogenesis, Feammox, and Denitrification in a Single Reactor Treating Fish Sludge

An integrated anaerobic digestion system for the simultaneous removal of carbon and nitrogen from fish sludge was developed by coupling iron sludge supplementation with intermittent aeration. In terms of nitrogen removal, Fe(III) in iron sludge could trigger Feammox reactions and intermittent aeration could drive the Fe(II)/Fe(III) cycle to sustain continuous ammonia removal. Mass balance analysis suggested that nitrate was the main product of Feammox, which was subsequently removed through heterotrophic denitrification. In terms of carbon removal, the Fe(III)-induced dissimilatory iron reduction (DIR) process significantly promoted fish sludge hydrolysis and provided more simple organics for methanogens and denitrifiers, but aeration showed a negative impact on methanogenesis. To promote nitrogen removal and avoid serious methanogenesis inhibition, different aeration intensities were studied. Results showed that compared with the control without aeration or iron sludge addition, aeration for 5 min every 3 days (150 mL/min) contributed to a 29.0% lower NH4+-N concentration and a 12.1% lower total chemical oxygen demand level on day 28, and the decline in methane yield was acceptable (only 13.5% lower). Simultaneous methanogenesis, Feammox, and denitrification in a single reactor treating fish sludge were achieved, which provides a simple and low-cost strategy for the treatment of organic wastewater.

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.

Enhancement and mechanisms of iron-assisted anammox process

Anaerobic ammonium oxidation (anammox) is a sustainable biological nitrogen removal technology that has limited large-scale applications owing to the low cell yield and high sensitivity of anammox bacteria (AnAOB). Fortunately, iron-assisted anammox, being a highly practical method could be an effective solution. This review focused on the iron-assisted anammox process, especially on its performance and mechanisms. In this review, the effects of iron in three different forms (ionic iron, zero-valent iron and iron-containing minerals) on the performance of the anammox process were systematically reviewed and summarized, and the strengthening effects of Fe (II) seem to be more prominent. Moreover, the detailed mechanisms of iron-assisted anammox in previous researches were discussed from macro to micro perspectives. Additionally, applicable iron-assisted methods and unified strengthening mechanisms for improving the stability of nitrogen removal and shortening the start-up time of the system in anammox processes were suggested to explore in future studies. This review was intended to provide helpful information for scientific research and engineering applications of iron-assisted anammox.