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

The trial was conducted at a Westernport Water wastewater treatment plant on Phillip Island by scientists from RMIT University, Westernport Water and Australia’s national science agency, CSIRO. The team installed a 330-square-metre floating wetland—roughly the size of one and a half tennis courts—in one wastewater holding lagoon and compared its performance with a similar control lagoon that had no floating platform. Both lagoons continued operating under normal plant conditions, allowing the researchers to observe how the wetland performed in a real treatment environment rather than in a laboratory tank.

Wastewater treatment is an overlooked source of climate pollution. Globally, the sector is estimated to account for about 1.6% of human-induced greenhouse gas emissions. High concentrations of organic matter and nutrients such as nitrogen and phosphorus create ideal conditions for microbial communities. Some of these microbes generate methane, while others produce nitrous oxide and other gases during the biological processes used to break down pollutants. Because wastewater facilities must treat these nutrient-rich flows continuously, even relatively small changes in emissions intensity can have a substantial effect at national and global scales.

The floating wetland changed the biological environment at the surface of the lagoon. Its plants were rooted in a buoyant platform, while their root systems extended directly into the wastewater below. These submerged roots provided a vast, oxygen-variable habitat for bacteria and other microorganisms. Such microbial communities can consume dissolved nutrients and organic compounds, transforming them into less harmful forms or incorporating them into plant growth. The roots also create microscale zones where oxygen-rich and oxygen-poor reactions occur side by side, potentially shifting the balance away from microbes that generate greenhouse gases and toward those that consume or avoid producing them.

The measured reductions were substantial. Compared with the untreated control lagoon, the lagoon fitted with the floating wetland recorded carbon dioxide emissions reductions of up to 36% and methane reductions of up to 66%. Nitrogen emissions fell by as much as 18%. When the results were considered across the lagoon’s emissions profile, the researchers reported an overall greenhouse gas reduction of more than 25%. The improvements did not appear immediately: the strongest changes became evident after approximately four months, suggesting that the plant roots and their associated microbial communities needed time to establish and mature.

To track those changes, the researchers continuously monitored emissions from both lagoons over the full two-year study. They used “Pondi,” a solar-powered environmental sensor developed by RMIT and collaborating institutions, including Leading Edge Engineering Solutions, Deakin University and The University of Queensland. Continuous monitoring is important because gas emissions from wastewater ponds can vary with temperature, sunlight, rainfall, wind, organic loading and seasonal biological activity. Rather than relying on occasional samples, the sensor system allowed the team to observe emission patterns over extended periods and compare the floating-wetland lagoon with the control under changing operational conditions.

Study first author Dr Lukas Schuster of RMIT University said the results demonstrate that floating wetlands can do more than remove nutrients from wastewater. “This is the first time we’ve had evidence on this scale that supporting microbial communities in the root systems of wetland plants can reduce wastewater emissions without relying on high-tech solutions,” Schuster said. He described the findings as a strong example of how nature-based systems could turn wastewater treatment into part of the climate solution. The plants do not replace the treatment process, but add a biological layer that can influence the chemistry and microbiology of the lagoon.

The approach also offers a practical advantage for water utilities: the platforms can be retrofitted into existing lagoons. That means operators may be able to reduce emissions without rebuilding ponds, installing complex gas-treatment equipment or significantly changing established treatment processes. Westernport Water Managing Director Dona Tantirimudalige said the findings provide a foundation for testing the technology across different lagoon designs and operating conditions. She noted that utilities need credible and cost-effective ways to meet emissions targets while maintaining reliable wastewater services, particularly in regional areas where capital and energy resources can be limited.

CSIRO is now developing and applying floating wetlands through its nature-based solutions program. In addition to reducing greenhouse gas emissions, the systems may capture or transform contaminants before treated water is released into downstream ecosystems. Their ecological value could extend beyond water quality: floating vegetation can provide habitat for microorganisms, invertebrates and some wildlife, although the trial required ongoing maintenance, including weed control and measures to deter birds from damaging the plants. The researchers emphasize that performance will depend on factors such as plant selection, platform coverage, lagoon design, wastewater composition and local climate.

The Phillip Island project is already inspiring a broader experiment. RMIT researchers, working with Melbourne Water and the Bass Coast Landcare Network, are testing floating wetlands in farm dams across the Bass Coast region. That work is examining water quality, biodiversity and emissions reductions, with the possibility of applying the technology to some of Australia’s approximately 1.8 million farm dams. If the systems perform reliably in agricultural environments, they could become a flexible tool for managing nutrient pollution and supporting biodiversity across landscapes far beyond wastewater plants. The wastewater study, titled “Constructed floating wetlands cut greenhouse gas emissions from wastewater lagoons,” appears in the Journal of Environmental Management and marks a significant step toward making engineered wetlands a measurable component of climate mitigation.

Subject of Research: Not applicable

Article Title: Constructed floating wetlands cut greenhouse gas emissions from wastewater lagoons

News Publication Date: 15-Aug-2026

Web References: https://bg.copernicus.org/articles/22/5051/2025/; https://www.csiro.au/en/research/natural-environment/water/constructed-floating-wetlands; https://www.rmit.edu.au/news/all-news/2026/apr/floating-wetlands-bass-coast; https://www.rmit.edu.au/research/centres-collaborations/centre-for-nature-positive-solutions

References: Journal of Environmental Management. DOI: 10.1016/j.jenvman.2026.130663

Image Credits: RMIT University

Keywords: floating wetlands, wastewater treatment, greenhouse gas emissions, methane, carbon dioxide, microbial communities, nature-based solutions, water management, sustainable agriculture, aquatic ecosystems

Addressing these challenges, a pioneering team of researchers from Radboud University in the Netherlands has developed an ultra-broadband coherent open-path spectroscopy (COPS) system designed to revolutionize real-time gas monitoring in wastewater treatment environments. This innovative instrument utilizes a mid-infrared light source with an unparalleled spectral bandwidth ranging approximately from 2 to 11.5 micrometers. The broad spectral range enables simultaneous, high-resolution detection of multiple gases, including methane, carbon dioxide, nitrous oxide, ammonia (NH₃), carbon monoxide (CO), and water vapor (H₂O). Unlike traditional methods reliant on discrete point measurements, the COPS system captures the integrated concentration profiles of gases over extended atmospheric paths in real time, providing a highly sensitive and temporally resolved picture of emissions.

The deployment of the COPS system atop an aeration tank at a Dutch WWTP exemplifies its advanced capabilities. Methane and carbon dioxide, both key indicators of organic matter decomposition and process aeration efficiency, were continuously monitored. The system revealed clear correlations between aeration schedules and fluctuations in gas concentrations, underscoring its potential to directly inform operational adjustments to mitigate emissions. Notably, nitrous oxide and ammonia levels remained relatively stable during the observation period, providing further insight into emission dynamics that are typically difficult to capture with conventional methods.

Technically, the COPS system operates by transmitting a coherent mid-infrared laser beam across an open atmospheric path above the wastewater treatment tank. As the light traverses this path, it interacts with airborne gas molecules, which absorb specific wavelengths corresponding to their unique vibrational and rotational transitions. The system’s detectors analyze the absorption spectra with high precision, enabling quantitative determination of multiple gas concentrations simultaneously. This coherent spectroscopy approach maximizes signal-to-noise ratios and improves sensitivity well beyond classical optical absorption techniques, such as non-dispersive infrared sensors or tunable diode laser absorption spectroscopy.

Beyond improving sensitivity and temporal resolution, the open-path design of the COPS system addresses limitations inherent in traditional point samplers. Point sensors provide data representative of gas concentrations at a fixed location, often failing to capture emissions dispersed over large or spatially complex sites. The COPS system’s extended beam path effectively averages emissions over an area, resulting in a more cohesive and comprehensive understanding of gaseous outputs. This spatial integration is particularly advantageous in WWTPs where emission sources—including open tanks, sludge storage, and aeration basins—are distributed and dynamically changing.

The significance of this technology extends beyond academic interest into practical environmental management and regulatory compliance spheres. Real-time analytics afforded by the COPS system enable WWTP operators to identify emission spikes immediately and evaluate the effectiveness of operational changes or mitigation technologies. With enhanced emissions quantification, facilities can more accurately report environmental performance and meet increasingly stringent regulatory standards. This can further guide long-term strategies to reduce greenhouse gas footprints and promote sustainability within the wastewater sector.

Dr. Simona Cristescu, a leading analytical chemist and co-developer of the COPS system, highlights the transformative impact of this breakthrough: “By enabling simultaneous, precise detection of a multitude of greenhouse gases with negligible interferences, our system offers a leap forward in our ability to monitor and understand emissions from complex industrial sites. This capability empowers more informed decisions towards emission reduction and sustainability.”

The research exemplifies a successful collaboration between academia and industry stakeholders, leveraging state-of-the-art laser technology and environmental science. Funding support from the EU Horizon2020 TRIAGE Project and Dutch water authorities underlines the priority of developing robust solutions for environmental monitoring challenges. The study’s findings, published in the journal Environmental Science and Ecotechnology, showcase not only technological innovation but also the potential for scalable applications across other sectors burdened by complex emission profiles.

Industrial manufacturing, agricultural facilities, and even atmospheric science research stand to benefit from this spectral monitoring advancement. As the technology matures, adaptations could enable remote sensing of greenhouse gases at regional scales, offering policymakers and environmental agencies a powerful tool to verify emission inventories and support climate action plans. The ability to conduct continuous, non-invasive, and multi-gas monitoring with minimal maintenance and operational overhead makes the COPS approach particularly appealing for diverse deployment scenarios.

However, challenges remain to fully integrate this technology into routine operational frameworks. Calibration protocols, data interpretation algorithms, and cost scalability must be further refined to ensure widespread adoption. Additionally, integrating COPS measurements with digital twins and process control systems could unlock real-time feedback loops, optimizing emission management strategies dynamically. Such advancements would position WWTPs and related industries at the forefront of green technology adoption.

In conclusion, the ultra-broadband coherent open-path spectroscopy system represents a watershed moment in environmental gas monitoring, bridging the gap between laboratory-grade analytical precision and field applicability. This real-time multi-gas detection platform not only advances scientific understanding of emissions from wastewater treatment but also lays the groundwork for smarter, sustainable industrial practices worldwide. As environmental pressure intensifies and regulatory landscapes evolve, innovations like the COPS system will be indispensable in achieving meaningful greenhouse gas mitigation and environmental stewardship.

Subject of Research: Not applicable

Article Title: Ultra-broadband coherent open-path spectroscopy for multi-gas monitoring in wastewater treatment

News Publication Date: 17-Mar-2025

Web References: http://dx.doi.org/10.1016/j.ese.2025.100554

References: 10.1016/j.ese.2025.100554

Image Credits: Environmental Science and Ecotechnology

Keywords: Environmental monitoring