A recent study published in Bioengineering presents an innovative waste-to-value strategy that transforms two abundant waste streams into valuable agricultural and industrial products.
Study: From Waste to Value: Urine and Ash as Sustainable Sources for Green Ammonia and Calcium Phosphate Fertilizers. Image Credit: Yasni/Shutterstock.com
The paper explains a process that produces green ammonia and calcium phosphate fertilizers with high recovery efficiencies, offering an environmentally friendly alternative to conventional fertilizer production.
Transforming Two Waste Streams into Valuable Resources
The growing demand for sustainable fertilizers and green chemicals has increased interest in resource recovery technologies. These technologies focus on extracting value from waste. Most nutrient-rich waste streams from agriculture and the biomass industry remain underutilized. Urine contains high levels of nitrogen and phosphorus, and wood ash provides a valuable source of calcium and other minerals. Conventional waste management often fails to recover these resources, allowing them to contribute to nutrient pollution or be discarded in landfills.
Several technologies can recover either nitrogen or phosphorus from urine. However, many of these methods require high energy input. They also depend on costly equipment or complex operating conditions. Wood ash also contains useful minerals. Despite this, its use in nutrient recovery remains limited. These challenges highlight the need for a simple and scalable solution.
Researchers developed the "urinash process." This process combines biological and chemical reactions that convert waste into valuable products. It first converts nitrogen in urine into green ammonia. Then, it combines the remaining phosphate with calcium extracted from wood ash. This reaction produces calcium phosphate fertilizer. It also reduces waste and supports a circular approach to fertilizer production.
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Building the Integrated Urinash Process
The researchers evaluated the urinash process using a multipurpose artificial urine that closely mimics the composition of natural urine while providing consistent experimental conditions. They tested two methods to convert urea into ammonia. One method used the commercially available urease enzyme, while the other relied on the naturally occurring urease-producing bacterium Providencia rettgeri, isolated from household urine scale.
The team monitored ammonia production, phosphate concentration, bacterial growth, citrate consumption, and pH throughout the biological conversion process. They combined spectroscopic techniques, mass spectrometry, and microbiological analyses to track these changes and assessed ammonia recovery through simple distillation after biological treatment.
The researchers characterized wood ash collected from a biomass heating facility using several analytical techniques and quantified the amount of calcium recovered. They also tested different hydrochloric acid concentrations to identify the most effective conditions for calcium extraction.
In the final stage, the researchers combined the calcium-rich ash extract with nitrogen-depleted urine under controlled conditions to produce calcium phosphate. They then analyzed the recovered products and remaining by-products to evaluate resource recovery efficiency, product composition, and the overall performance of the integrated process.
Efficient Recovery of Green Ammonia and Fertilizer Products
Both biological approaches efficiently converted urea into ammonia, achieving recovery efficiencies of approximately 68%. The urease-based process completed the conversion within a few hours, whereas the bacterial process required a longer fermentation period but offered greater potential for large-scale implementation. As fermentation progressed, ammonia production increased the solution pH, while Providencia rettgeri also consumed citrate, removing additional organic compounds from the urine.
After biological conversion, the researchers recovered ammonia through simple distillation. The nitrogen-depleted urine retained most of its phosphorus, making it an ideal feedstock for fertilizer production. Acid treatment released substantial amounts of soluble calcium from wood ash, with 5 M hydrochloric acid delivering the highest extraction efficiency. When the researchers combined the calcium-rich extract with the treated urine, the reaction recovered nearly all the available phosphate as calcium phosphate, achieving a precipitation efficiency of 99%.
Overall, the integrated process recovered around 74% of the soluble calcium from wood ash. In addition to the main products, the process generated several potentially useful by-products, including bacterial biomass, magnesium-rich urine scale, a silicate-rich ash residue, and a mineral-rich brine.
According to the researchers, large-scale implementation of the urinash process in Germany could produce hundreds of thousands of tonnes of calcium phosphate fertilizer each year. It could also generate substantial quantities of green ammonia, reducing reliance on conventionally produced fertilizers and ammonia. Such an approach could reduce dependence on conventionally manufactured fertilizers and ammonia, improve nutrient recovery, and help mitigate environmental pollution.
A Practical Step Toward Circular Fertilizer Production
The study highlights the potential of an integrated circular economy approach to transform two common waste streams into valuable agricultural and industrial resources. By recovering nitrogen, phosphorus, and calcium from animal urine and wood ash, the urinash process addresses both nutrient pollution and the growing demand for sustainable raw materials.
Green ammonia produced through this process offers a more sustainable alternative to the energy-intensive Haber–Bosch process, while the recovered calcium phosphate could reduce dependence on environmentally damaging phosphate mining. Beyond fertilizer production, both products have potential applications across the chemical, energy, and manufacturing sectors.
Future research should validate the technology using real animal and human urine, evaluate the impact of pharmaceuticals and heavy metals, and optimize acid consumption and ammonia recovery. It should assess its economic and environmental performance through comprehensive techno-economic and life-cycle analyses. Scaling up the process will also require continuous production systems and efficient waste collection infrastructure.
Overall, the study establishes a promising foundation for integrated waste-to-value technologies. By converting abundant waste streams into high-value products, the urinash process offers a practical pathway toward cleaner fertilizer production, improved nutrient recycling, and more sustainable circular resource management.
Journal Reference
Li, Z., Tiganescu, E., et al. (2026). From Waste to Value: Urine and Ash as Sustainable Sources for Green Ammonia and Calcium Phosphate Fertilizers. Bioengineering, 13, 720. https://www.mdpi.com/2306-5354/13/7/720
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