The motivation is straightforward. Conventional biological nitrogen removal relies on two sequential microbial steps: aerobic nitrification, in which ammonium is oxidized to nitrate, and anoxic denitrification, in which nitrate is reduced to nitrogen gas. The first step demands massive aeration, often accounting for a large share of a treatment plant’s electricity budget, while the second requires external organic carbon, typically methanol or other dosed chemicals, because municipal wastewater after primary treatment often lacks sufficient biodegradable carbon. Both steps also generate nitrous oxide, a greenhouse gas roughly 300 times more potent than carbon dioxide. As utilities worldwide face pressure to cut energy use, chemical consumption, and emissions simultaneously, autotrophic processes that sidestep aeration and organic carbon dosing have become a central goal of environmental biotechnology, and iron redox cycling offers several routes toward that goal.

The review identifies four principal iron-coupled nitrogen conversion pathways. The first and most celebrated is Feammox, the anaerobic oxidation of ammonium coupled to the reduction of Fe(III) to Fe(II). Discovered in soils and sediments, including paddy fields and wetland environments, Feammox means ammonium, long considered inert without oxygen, can be oxidized under anoxic conditions when ferric iron serves as the electron acceptor. The second pathway is nitrate-dependent ferrous oxidation, often abbreviated NDFO, in which microorganisms oxidize Fe(II) to Fe(III) while reducing nitrate to nitrogen gas, effectively achieving denitrification with iron rather than organic carbon as the electron donor. The third is iron-enhanced anaerobic ammonium oxidation, or anammox, in which appropriate iron dosing stimulates the anammox bacteria that combine ammonium and nitrite directly into nitrogen gas. The fourth is chemodenitrification, a purely abiotic reaction in which Fe(II) chemically reduces nitrite, producing nitric oxide, nitrous oxide, or nitrogen gas along with ferric oxide minerals.

What distinguishes this review from earlier surveys, the authors argue, is its insistence that these pathways cannot be treated in isolation. In a real bioreactor, Fe(II) and Fe(III) coexist, nitrite and nitrate are both present, and the microbial community contains organisms capable of several different metabolisms at once. The result is intense competition for shared substrates. Feammox products can feed anammox, but the two processes can also compete for ammonium. Chemodenitrification can consume the nitrite that anammox bacteria need, or it can generate the nitrous oxide that plants are trying to eliminate. Kinetic mismatch compounds the problem: abiotic Fe(II)-nitrite reactions can be fast, while microbial iron reduction and oxidation are comparatively slow, so the balance between biological and chemical routes shifts with operating conditions in ways that are difficult to predict. Understanding this network of competing and cooperating reactions, rather than optimizing any single pathway, is presented as the key to stable system performance.

Iron bioavailability emerges as a central control variable. Ferric iron is poorly soluble at neutral pH, and much of it in a reactor exists as solid oxyhydroxide minerals such as ferrihydrite, goethite, and hematite. Microorganisms can only use the fraction they can access, whether by directly reducing soluble Fe(III) species, by attaching to mineral surfaces, or by using electron shuttles such as humic substances or activated carbon that ferry electrons between cells and insoluble oxides. Studies cited in the review show that ferrihydrite, the most amorphous and reactive ferric mineral, can markedly improve Feammox performance in continuous reactors, while more crystalline oxides are far less effective. Chelating agents can keep Fe(II) in solution for nitrate-dependent ferrous oxidation, and sponge iron or zero-valent iron has been used as a slow-release source of ferrous ions that sustains iron-driven denitrification over long operating periods. The choice of iron form, dose, and delivery strategy therefore determines whether the iron cycle turns productively or grinds to a halt.

The review also examines the operational levers that engineers can actually turn. Iron concentration and speciation sit at the top of the list, because moderate Fe(II) additions are known to accelerate anammox growth, likely by supporting the synthesis of iron-rich enzymes such as the heme-containing enzymes of anammox metabolism, whereas excessive doses can inhibit activity or trigger mineral precipitation that buries biomass. Dissolved oxygen must be carefully managed: too much oxygen chemically oxidizes Fe(II) and favors conventional nitrification, but micro-oxygen conditions have recently been shown to stimulate Feammox, suggesting a narrow optimal window. pH governs both the solubility of iron minerals and the rate of chemodenitrification, which is strongly pH-dependent, and temperature shapes the microbial community, with cold conditions known to suppress anammox consortia. Electron-transfer mediators, from natural humics to engineered conductive materials, offer a further handle for accelerating electron flow between microbes and iron minerals.

Mineral encrustation is one of the most stubborn translational barriers the review highlights. When Fe(II) is oxidized, whether biologically or chemically, the resulting ferric oxides precipitate as fresh mineral phases that can coat cell surfaces and biofilm matrices. Encrusted cells lose access to substrates and electron acceptors, and reactor performance decays. Nitrite, a common intermediate in all of these systems, plays a double role here, because it both drives abiotic Fe(II) oxidation and promotes the cell encrustation observed in nitrate-reducing iron-oxidizing cultures. Managing where and how fast ferric minerals form, and preventing them from smothering the microbial catalysts, is identified as a prerequisite for long-term reactor stability. On the positive side, the secondary minerals produced during chemodenitrification have been shown to sequester other contaminants such as arsenate, hinting at co-benefit opportunities in waters with mixed pollution.

Greenhouse gas risk is another theme the authors refuse to gloss over. Chemodenitrification is a recognized source of nitrous oxide, and its contribution depends strongly on pH and on the relative rates of the abiotic and biological branches of the nitrogen network. Earlier work suggests that abiotic nitrous oxide production, while strongly pH-dependent, may contribute little to total emissions in conventional biological nitrogen removal systems, but iron-amended reactors change the chemistry substantially, and the review stresses that pathway partitioning, meaning how much nitrogen flows through Feammox, anammox, NDFO, and chemodenitrification respectively, remains uncertain in most systems. Without reliable tools to quantify that partitioning, engineers cannot yet guarantee that an iron-based process will meet increasingly strict emissions limits. Isotopic tracing techniques, including nitrogen-15 labeling, are among the methods being deployed to resolve these questions, and recent studies have used them to distinguish anammox-dominated from Feammox-dominated regimes.

The review closes with a set of research priorities intended to move iron redox-coupled nitrogen conversion from promising to practical. The unresolved microbial mechanisms come first: the organisms responsible for Feammox, including the characterized Acidimicrobiaceae strain A6, are still poorly understood, and their kinetics, yields, and ecological interactions need systematic quantification. Slow reaction kinetics must be overcome, whether through better inocula, optimized mineral substrates, electron shuttles, or reactor designs that concentrate active biomass. Robust models that predict pathway partitioning under real wastewater conditions are needed so that operators can steer the network toward nitrogen gas rather than nitrous oxide. Strategies that lower iron demand, such as coupling Feammox with anammox so that recycled Fe(II) is reoxidized internally, are highlighted as routes to high-level nitrogen removal with minimal chemical input. If those gaps can be closed, the authors argue, iron redox cycling could underpin a new generation of low-energy, low-carbon nitrogen removal technologies, turning one of the planet’s most common elements into an ally in the effort to make wastewater treatment genuinely sustainable.

Subject of Research: Iron redox-coupled nitrogen conversion pathways for sustainable wastewater treatment

Article Title: A critical review of iron redox-coupled nitrogen conversion for sustainable wastewater treatment

Article References: Guo, R., Cen, X., Li, C., Zuo, Z., & Zheng, M. (2026). A critical review of iron redox-coupled nitrogen conversion for sustainable wastewater treatment. ENGINEERING Environment, 20(12), Article 188. https://doi.org/10.1007/s11783-026-2288-4

Image Credits: AI Generated

DOI: 10.1007/s11783-026-2288-4

Keywords: iron redox cycling, Feammox, anammox, nitrate-dependent ferrous oxidation, chemodenitrification, nitrogen removal, wastewater treatment, autotrophic denitrification, nitrous oxide emissions, electron shuttles, mineral encrustation, sustainable engineering

The core of the innovation lies in combining two well-known but notoriously difficult-to-pair processes. The first is anammox, short for anaerobic ammonium oxidation, in which specialized bacteria convert ammonium and nitrite directly into inert nitrogen gas without oxygen or organic carbon. The second is sulfide-dependent autotrophic denitrification, or S-SADN, in which sulfur-oxidizing bacteria use sulfide as an electron donor to reduce nitrate and nitrite. Each process alone has limitations: anammox bacteria are slow-growing and sensitive to sulfide toxicity, while conventional autotrophic denitrification with sulfide generates excessive sulfate and competes with anammox for nitrite. The new work shows that a carefully tuned mixotrophic design, in which the denitrifying partner also consumes a small amount of organic carbon, resolves these conflicts.

Lead author Yifan Zhang and colleagues integrated an anammox-enriched culture designated KAS1 with Thauera sp. AutoDN2, a sulfide-oxidizing denitrifying bacterium previously identified by the same group. Crucially, AutoDN2 is not a strict autotroph; it can use both sulfide and acetate, allowing the researchers to maintain a very low carbon-to-nitrogen ratio of just 0.8. Under these conditions, the coupled system removed 98.1 percent of ammonium and 99.4 percent of total nitrogen, performance figures that rival or exceed the best reported values for similar coupled systems while requiring far less external carbon than heterotrophic denitrification would demand.

Long-term operation in fed-batch mode revealed how the workload was divided between the two guilds. Anammox accounted for 71.2 to 77.1 percent of the total nitrogen removed, confirming that it remained the dominant pathway throughout extended operation. The mixotrophic S-SADN component provided a complementary route, polishing nitrate produced by anammox and handling sulfide oxidation. This division of labor proved stable over repeated feeding cycles, a critical finding because many attempted couplings of anammox with sulfur-driven denitrification have collapsed under sulfide inhibition or nitrite starvation of the anammox population.

One of the most striking outcomes concerns sulfate, the typical end product of sulfide oxidation. In conventional sulfide-based autotrophic denitrification systems, sulfide is fully oxidized to sulfate, which accumulates in the effluent, corrodes infrastructure, and raises salinity in receiving waters. In the mixotrophic system, sulfate yields were 63 to 68 percent lower than in purely autotrophic counterparts. The mechanism appears to be stoichiometric: when acetate is available, the denitrifiers require less sulfide per unit of nitrate reduced, and the reduced sulfide oxidation load shifts the sulfur balance away from complete oxidation. In effect, the organic co-substrate absorbs part of the electron-donation burden that sulfide would otherwise carry alone.

To verify that both processes were genuinely active rather than merely coexisting, the researchers tracked transcript levels of key functional genes. Stable expression of hzsA and hzsB, which encode hydrazine synthase subunits essential to the anammox metabolism, demonstrated that the anammox bacteria maintained their central catabolic machinery. Simultaneously, steady transcription of narG and napA, genes encoding nitrate reductases in the denitrification pathway, confirmed that AutoDN2 was actively respiring nitrogen oxides. The synchronized activity of these gene sets provides molecular evidence of metabolic synergy rather than competitive exclusion, and it suggests the partnership could be monitored in real time at full-scale plants through transcriptomic or genomic surveillance of activated sludge.

Equally notable is what the system did not emit. Across the experimental campaign, the researchers detected no nitrous oxide, a greenhouse gas roughly 265 times more potent than carbon dioxide over a century and a notorious byproduct of conventional nitrification-denitrification. The authors attribute this to the high nitrite affinity of anammox bacteria, which scavenge nitrite so efficiently that denitrifiers are rarely pushed toward the nitric oxide reductase steps that leak nitrous oxide. For utilities facing tightening greenhouse gas accounting rules, an ammonium treatment train that emits essentially no nitrous oxide represents a significant compliance advantage alongside its energy savings.

The practical implications extend to several wastewater streams where sulfide and ammonium co-occur with little biodegradable carbon. Anaerobic digester liquors, landfill leachate, tannery effluents, petrochemical wastewater, and sidestream returns from sludge treatment all fit this profile. In such streams, sulfide is usually treated as a nuisance to be stripped or precipitated before biological nitrogen removal, adding cost and complexity. The coupled platform instead treats sulfide as a free electron donor, converting a pollutant into a process resource. Because anammox does not require aeration and the denitrifying partner needs only a whisper of organic carbon, the system avoids the aeration and carbon-dosing costs that dominate conventional treatment economics.

The authors caution that translating a fed-batch laboratory model to continuous full-scale operation will require attention to process control, particularly maintaining the delicate nitrite balance that both guilds depend upon and managing sulfide loading to keep concentrations below anammox inhibition thresholds. Nevertheless, the demonstration that strategic mixotrophy can simultaneously mitigate sulfide toxicity, suppress sulfate overproduction, stabilize integrated carbon-nitrogen-sulfur cycling, and deliver near-complete nitrogen removal marks a substantial advance. As water utilities worldwide seek low-carbon pathways to meet stricter nitrogen discharge limits, this anammox-mixotrophic denitrification partnership offers a compelling blueprint: two microbial metabolisms, each compensating for the other’s weaknesses, working in concert to turn some of wastewater’s most stubborn pollutants into harmless nitrogen gas.

Subject of Research: Coupling anammox with mixotrophic sulfide-driven autotrophic denitrification for extensive biological nitrogen removal from sulfide-rich, carbon-limited wastewater

Article Title: Synergistic coupling of anammox and mixotrophic sulfide-driven denitrification for extensive nitrogen removal

Article References: Synergistic coupling of anammox and mixotrophic sulfide-driven denitrification for extensive nitrogen removal. (n.d.). https://doi.org/10.1007/s11783-026-2281-y

Image Credits: AI Generated

DOI: 10.1007/s11783-026-2281-y

Keywords: anammox, autotrophic denitrification, sulfide oxidation, wastewater treatment, nitrogen removal, Thauera, mixotrophic metabolism, nitrous oxide, sulfate reduction, carbon-nitrogen-sulfur cycling, environmental biotechnology, low-carbon sanitation