Farmers rely on potassium to support plant growth, water regulation, and crop resilience. Today, most agricultural potassium comes from mined potash, whose extraction can generate saline waste and damage freshwater ecosystems. Concentrated global supply also exposes fertilizer markets to geopolitical and economic disruption. Wastewater contains recoverable potassium, but wastewater treatment plants typically prioritize nitrogen and phosphorus, leaving much of this nutrient unused.
Previous studies have tested precipitation, membranes, and selective separation, yet their findings remain scattered across different waste streams and experimental conditions. Many experiments use artificial solutions rather than the complex mixtures handled by wastewater treatment plants. Given these challenges, a deeper investigation is needed into which wastewater sources, recovery technologies, and fertilizer products can work together in practice.
Researchers Cora Eichholz and Heidrun Steinmetz from the Department of Resource-Efficient Wastewater Treatment at RPTU Kaiserslautern-Landau in Germany report (DOI: 10.1016/j.ese.2026.100777) their findings in Environmental Science and Ecotechnology in a paper accepted on October 8th, 2026. Examining 110 publications, the review connects potassium concentrations, recovery processes, product forms, and barriers to practical deployment, showing why wastewater composition should guide fertilizer design rather than treating all nutrient-rich effluents as interchangeable sources.
The researchers followed the Joanna Briggs Institute (JBI) scoping-review framework, searching Scopus and expanding the evidence base through citation-network analysis. They classified 110 publications by wastewater origin, treatment process, technology maturity, and resulting fertilizer. The evidence revealed a field still dominated by laboratory work: 61 studies reported laboratory-scale experiments, compared with 11 at pilot scale, and approximately 34% relied on synthetic solutions. Among 94 publications examining specific technologies, potassium-struvite precipitation appeared in 40, electrodialysis in 17, and ion exchange in 15.
Wastewater composition emerged as a decisive factor. Source-separated urine contained approximately 900-2,440 milligrams of potassium per liter, while liquid digestate contained approximately 1,494-2,440 milligrams per liter. Their nutrient-rich composition favors solid compound fertilizers such as potassium struvite. By contrast, primary-settled municipal wastewater contained about 25 milligrams per liter, suggesting a possible route to liquid or single-nutrient products after suitable concentration and separation.
Promising results also came with limitations. Across individual studies, reported potassium recovery ranged from 15% to 98% for potassium-struvite precipitation and 13% to 95% for electrodialysis, but these figures were obtained under differing conditions and are not directly comparable. Ammonium and sodium can compete with potassium, while membrane fouling, mineral scaling, energy requirements, and uncertain fertilizer purity complicate real-world deployment.
The authors said the central lesson is to start with the fertilizer farmers need, then work backward to the wastewater source and recovery process. A technology that performs well with a clean laboratory mixture may behave very differently when faced with fluctuating nutrient levels and competing ions in real wastewater, they explained. They also emphasized that recovering potassium is only part of the challenge: the final product must be safe, usable, and consistent enough for agricultural markets. In their assessment, sustained pilot testing and careful product-quality evaluation are essential before wastewater-derived potassium fertilizers can become a dependable option.
The review points toward wastewater treatment plants functioning not only as pollution-control facilities but also as nutrient-recovery hubs. Concentrated side streams could support production of solid, multi-nutrient fertilizers, while selected dilute streams might supply liquid potassium formulations for controlled cultivation systems, including hydroponics. However, the authors stress that practical use depends on reliable contaminant screening, especially for heavy metals and organic micropollutants, and compliance with the European Union Fertilising Products Regulation. Future work should test integrated treatment sequences using actual, variable wastewater, assess equipment performance over extended periods, and apply life-cycle assessment to environmental impacts. These steps could establish whether potassium recovery delivers benefits beyond technical feasibility.