Turning Urine Into Fertilizer: Scientists Unlock Sequential Recovery of Nitrogen, Potassium, and Phosphorus

Human urine has long been dismissed as waste, flushed away without a second thought. Yet each person excretes a remarkable concentration of the three nutrients that feed the world’s crops: nitrogen, phosphorus, and potassium. A new study published in Clean Technologies and Environmental Policy by researchers from Istanbul Technical University and the University of Calabria demonstrates that these nutrients can be harvested one after another from source-separated urine, using a carefully choreographed sequence of mineral precipitation reactions. The work, led by Işık Kabdaşlı and Alessio Siciliano, offers one of the most complete blueprints yet for converting a problematic waste stream into a suite of crystalline fertilizers, and it reveals a subtle chemical rule that has frustrated earlier attempts to capture potassium from urine-rich liquors.

The premise behind urine source separation is simple but powerful. Rather than diluting urine into the vast volumes of municipal sewage, where its nutrients become expensive to recover, diverting it at the toilet keeps it concentrated. Urine contributes the majority of the nitrogen and a large share of the phosphorus and potassium in domestic wastewater while representing only about one percent of the total flow. If those nutrients could be extracted as clean mineral products, cities could simultaneously ease the burden on treatment plants, reduce eutrophication of waterways, and produce fertilizers with a far smaller carbon footprint than conventional synthetic alternatives derived from natural gas and mined rock phosphate.

The centerpiece of the new strategy is struvite, a magnesium ammonium phosphate mineral with the formula MgNH4PO4·6H2O. Struvite precipitation is a well-established technique in wastewater engineering, prized because it locks nitrogen and phosphorus into a slow-release fertilizer in a single step. In the study, the researchers dosed magnesium into both fresh urine and hydrolyzed urine, the latter produced by storing urine so that urease enzymes break urea down into ammonium and carbonate. At a pH of 9.5, both feedstocks yielded phase-pure struvite, confirmed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The recovery efficiency, however, depended strongly on which matrix was treated: hydrolyzed urine delivered roughly 95 percent recovery of ammonium nitrogen, while fresh urine reached only about 76 percent, a difference the authors attribute to the distinct solution chemistries of the two streams.

The real challenge came with potassium. Urine contains potassium at concentrations comparable to phosphorus, but capturing it as a crystalline potassium phosphate has proven notoriously difficult. Two candidate minerals exist: K-struvite (MgKPO4·6H2O), the potassium analog of conventional struvite, and hazenite (KNaMg2(PO4)2·14H2O), a rare biologically related phosphate mineral first identified at Mono Lake in California. The researchers attempted to coax these phases out of raw urine at elevated pH values between 10.1 and 10.5, even supplying excess magnesium and adjusting the phosphorus dose. The result was consistent and, at first glance, discouraging: X-ray diffraction showed that ordinary ammonium struvite remained the sole crystalline product in every trial.

That failure turned out to be the study’s most instructive finding. Ammonium, present in urine at far higher concentrations than potassium, simply outcompetes potassium for the magnesium phosphate lattice. The struvite family of minerals crystallizes preferentially around ammonium, and as long as ammonium dominates the solution, potassium-targeted phases never get the chance to form. The researchers demonstrated that this competition is not a marginal effect but a fundamental gatekeeper: crystalline K-struvite and hazenite could only emerge when the ammonium nitrogen concentration entering the precipitation reactor was driven below approximately 360 milligrams per liter. In other words, the key to unlocking potassium recovery is not more reagent or higher pH, but the deliberate removal of ammonium first.

Armed with this insight, the team restructured the process into a sequential scheme. In the first stage, struvite precipitation strips most of the nitrogen and phosphorus from the urine. The supernatant, now depleted in ammonium but still carrying potassium and residual phosphate, becomes the feed for a second precipitation stage targeting potassium. This two-step arrangement transformed the outcome. The second stage recovered roughly 92 percent of the potassium from hydrolyzed urine, equivalent to about 94 percent overall, and about 86 percent from fresh urine, or roughly 89 percent overall. X-ray diffraction revealed a clear shift in product identity: hazenite became the dominant crystalline phase, with K-struvite-type structures appearing alongside it, marking the transition the single-stage experiments could never achieve.

The final stage of the sequence addresses the phosphate that survives the first two rounds of precipitation. Here the researchers turned to a different chemistry altogether: vivianite, an iron(II) phosphate mineral with the formula Fe3(PO4)2·8H2O. Vivianite has attracted growing interest in recent years as a phosphorus recovery route because it forms readily under reducing conditions and has potential uses as a pigment or iron fertilizer precursor. By dosing ferrous iron into the remaining liquor, the team polished off the residual phosphate effectively, completing a three-stage cascade in which nitrogen, potassium, and phosphorus are each funneled into a distinct, harvestable mineral product.

The elegance of the approach lies in its division of labor. Each mineral captures a different slice of the nutrient spectrum: struvite takes nitrogen and much of the phosphorus, hazenite and K-struvite take potassium with the remaining phosphate, and vivianite mops up what is left. Because the stages are linked, with each supernatant feeding the next reactor, the process avoids the phase competition that undermines single-step designs. The characterization work matters here too. By pairing recovery data with XRD, SEM, and EDS evidence, the authors show not just how much nutrient was removed but exactly which mineral phases formed, a level of verification that distinguishes genuine crystallization from amorphous precipitation and gives downstream users confidence in the product’s composition.

The implications extend beyond the laboratory. Phosphorus is a finite resource concentrated in a handful of mining regions, potassium fertilizers depend on energy-intensive extraction, and nitrogen production via the Haber-Bosch process consumes roughly one to two percent of global energy. Meanwhile, nutrient runoff from wastewater drives algal blooms and dead zones in coastal waters. A technology that converts urine into struvite, hazenite, and vivianite addresses all of these pressures at once, and it fits naturally into the decentralized sanitation concepts gaining traction in Europe and elsewhere, where urine-diverting toilets and collection logistics are already being piloted. The finding that ammonium must be depleted below a defined threshold before potassium phases will crystallize gives engineers a concrete design rule for scaling the process.

Challenges remain before such cascades operate at full scale. Reagent costs for magnesium and iron salts, reactor design for handling variable urine compositions, and the market acceptance of novel fertilizer minerals all require further work, and the authors note that recovery behavior is strongly matrix-dependent, meaning real-world streams will demand adaptive control. Yet the study provides something the field has lacked: a demonstrated, chemically verified pathway through which the three macronutrients in human urine can be separated sequentially rather than sacrificed to competition. As cities search for circular solutions to nutrient management, the humble flush may soon be recognized as the first step in a fertilizer supply chain rather than the end of one.

Subject of Research: Sequential recovery of nitrogen, potassium, and phosphorus from source-separated human urine by staged mineral precipitation

Article Title: Sequential recovery of N, K, and P from source-separated human urine via struvite, K-struvite, hazenite, and vivianite precipitation

Article References: Kabdaşlı, I., Andreoli, T., Dilsizoglu-Akyol, N., Ozyildiz, G., & Siciliano, A. (2026). Sequential recovery of N, K, and P from source-separated human urine via struvite, K-struvite, hazenite, and vivianite precipitation. Clean Technologies and Environmental Policy, 28(10), Article 266. https://doi.org/10.1007/s10098-026-03616-3

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03616-3

Keywords: human urine, nutrient recovery, struvite, K-struvite, hazenite, vivianite, phosphorus recovery, potassium recovery, nitrogen recovery, source separation, precipitation crystallization, circular economy