3D-Printed Living Materials Supercharge Wastewater Bacteria to Achieve Complete Nitrogen Removal

Wastewater treatment plants are among the most energy-hungry pieces of urban infrastructure on the planet, and a large share of that energy is spent on one deceptively simple task: removing nitrogen. Ammonium and other nitrogen compounds flowing out of cities and industries must be converted into harmless nitrogen gas before treated water can be returned to rivers and lakes. For two decades, researchers have pinned their hopes on a remarkable group of microorganisms known as anammox bacteria, which can perform anaerobic ammonium oxidation, converting ammonium and nitrite directly into nitrogen gas without the costly aeration and organic carbon demands of conventional treatment. Yet despite their promise, anammox-based systems have been persistently undermined by a stubborn problem: nitrate accumulation that leaves too much nitrogen still dissolved in the effluent.

A study published in Nature Sustainability by Yinuo Liu, Yingxin Zhao and colleagues at Tianjin University now reports a strikingly elegant solution to this bottleneck, one that reads almost like science fiction. Instead of adding chemicals or redesigning reactors, the team used a 3D bioprinter to fabricate engineered living materials, or ELMs, in which anammox bacteria and their denitrifying partners are locked together in a precisely printed microarchitecture. Within these printed living structures, the microbes exchange metabolites so efficiently that the system achieved complete nitrogen removal, eliminating one hundred percent of nitrogen from the water, and did so without any external supply of organic carbon. When the researchers validated the approach with real wastewater, the performance held.

The core insight behind the work is fundamentally ecological rather than purely technological. Anammox bacteria, often abbreviated AnAOB, are notoriously slow growers with doubling times measured in days or even weeks, and they naturally produce nitrate as a byproduct of their metabolism. In an ideal system, denitrifying bacteria living alongside them would consume that nitrate, reducing it further to nitrogen gas and closing the nitrogen loop. This cross-feeding partnership exists in nature, but in conventional bioreactors it is fragile. Metabolites drift away in the flowing water before partner organisms can capture them, and the physical distance between anammox cells and denitrifiers dilutes the chemical conversation the two groups need to hold. The result is nitrate piling up in the effluent and treatment performance falling short of what the biology should theoretically allow.

The Tianjin team’s answer was to give the microbial community an architecture. They formulated a bioink composed of sodium alginate and cellulose, two abundant and biocompatible natural polymers, and loaded it with a concentrated anammox consortium. Using extrusion-based 3D printing, they deposited this living ink into defined three-dimensional structures that combine two properties that are usually difficult to reconcile: mechanical stability to survive the harsh conditions of a wastewater reactor, and an open, porous microstructure that lets water and substrates flow through while keeping the cells densely packed inside. The printed materials function simultaneously as scaffolds, as microbial incubators and as diffusion barriers that trap metabolites close to the cells that produce them.

Spatial confinement proved to be the decisive factor. Inside the printed ELMs, cell densities reached levels far higher than those achievable in suspended cultures, forcing microbial cells into intimate proximity. That proximity activated synergistic metabolic pathways that remained dormant or marginal in free-floating consortia. Using metagenomic and metabolomic analyses, the researchers traced a rich exchange economy between the anammox bacteria and key partner taxa, notably denitrifiers of the Opitutus genus. The partners traded extracellular polysaccharides, amino acids and essential cofactors, with each group supplying metabolites the other could not synthesize on its own. In effect, the printed material recreated the dense, chemically connected microenvironments of natural biofilms, but with a geometry designed by engineers rather than left to chance.

The performance gains were dramatic. Anammox systems are infamous for their long start-up periods, the slow weeks during which the bacterial community establishes itself before a reactor reaches useful treatment capacity. The printed ELMs cut start-up time by 71.43 percent, a reduction that could translate directly into faster commissioning of full-scale treatment facilities. More importantly, once running, the confined communities achieved complete nitrogen removal, converting ammonium and nitrate all the way to nitrogen gas without the addition of external organic carbon. That last point matters enormously for sustainability, because conventional denitrification requires organic carbon dosing, typically methanol or other electron donors, which adds cost, operational complexity and a carbon footprint of its own. A system that couples anammox to denitrification using internally recycled metabolites sidesteps that requirement entirely.

The study goes beyond engineering demonstration to probe the underlying mechanisms in detail. Metagenomic evidence revealed the genetic basis of the cross-feeding behaviors between AnAOB and Opitutus, showing how spatial confinement reshaped gene content and pathway activity within the community. Non-targeted metabolomics compared the chemical profiles of free anammox consortia and the printed ELMs, documenting the enriched pools of shared metabolites inside the confined structures. Together, these analyses support a coherent picture: the printed architecture does not simply hold cells in place, it actively rewires the metabolic network of the community, favoring mutualistic exchanges over competition and enabling the coupled anammox-denitrification chemistry that has long been the goal of the field.

The broader implications extend across environmental biotechnology and materials science. Engineered living materials are an emerging class of substances in which living cells are embedded within a fabricated matrix, endowing the material with biological functions such as catalysis, sensing or self-repair. Applying this concept to wastewater treatment represents one of its most consequential potential uses, because the scale of the problem is enormous. Nitrogen removal is a major contributor to global energy consumption and greenhouse gas emissions, and the world’s growing cities are generating ever larger volumes of nitrogen-rich sewage. A technology that makes anammox systems start faster, perform better and operate without carbon dosing could meaningfully shrink the environmental footprint of sanitation infrastructure worldwide.

Significant challenges remain before printed living materials flow through municipal treatment plants. The researchers’ experiments were conducted at laboratory scale, and scaling up 3D bioprinting to produce cubic meters of living material, rather than laboratory specimens, will require new manufacturing approaches. The long-term durability of the alginate-cellulose matrix under continuous loading, shear and fluctuating wastewater chemistry must be demonstrated, and the materials must ultimately be retrievable and replaceable within industrial reactors. Regulatory questions about deploying concentrated engineered microbial communities in open infrastructure will also need careful attention. Nevertheless, the study provides what the field has long sought: a viable, mechanistically grounded strategy for the rapid establishment and enhanced performance of anammox systems, validated with real wastewater and grounded in a deep understanding of microbial ecology.

What makes the work resonate beyond its immediate application is the way it reframes the relationship between fabrication technology and biology. For most of industrial history, engineers have built inert structures and asked biology to adapt to them. Here the logic is inverted: the structure is printed around the biology, shaped to amplify the cooperative behaviors that evolution has already written into the microbial genomes. The printed lattice becomes a kind of architectural mediator, translating the metabolic potential of anammox bacteria and their partners into a treatment process that is faster, cleaner and more complete than either organism group could deliver alone. If the approach survives the journey from bench to plant, the humble printed hydrogel may come to be seen as a quiet turning point in humanity’s effort to clean its own water, one layer of living material at a time.

Subject of Research: 3D-bioprinted engineered living materials that couple anammox bacteria and denitrifiers for complete nitrogen removal in wastewater treatment

Article Title: 3D-printed living materials for anammox–denitrification coupling in wastewater treatment

Article References: 3D-printed living materials for anammox–denitrification coupling in wastewater treatment. (n.d.). https://doi.org/10.1038/s41893-026-01921-9

Image Credits: AI Generated

DOI: 10.1038/s41893-026-01921-9

Keywords: anammox, denitrification, 3D bioprinting, engineered living materials, wastewater treatment, nitrogen removal, microbial cross-feeding, spatial confinement, bioink, sodium alginate, metabolomics, environmental biotechnology