The Urinash process: Combined utilisation of two waste streams
Researchers at Saarland University have developed a process that produces green ammonia and phosphate fertilizer from urine and wood ash. In their urinash process, they combine a series of simple biological and chemical processes.
The conversion of waste into valuable materials is vital for a circular economy. However, it is not uncommon that at least one additional substance must be added to a single waste stream in order to fully utilise its constituents. If this extra material is also recovered from waste, the entire process becomes even more sustainable.
Why should urine and ash be recycled together?
The urine for the Urinash process would come from industrial livestock farming. Combining it with wood ash is promising because both waste types are typically found in rural areas, allowing short-distance transport to a shared treatment site.
Animal urine – the liquid part of separated manure or slurry – is rich in nitrogen and phosphorus. However, its direct use as fertilizer on farmland is legally restricted in many countries. For example, time limits on application aim to protect groundwater reservoirs and rivers. Yet, these measures have only been partially effective. This is shown, for example, by the Federal Government’s 2024 Nitrate Report.
Furthermore, large industrial farms often produce larger amounts of manure than they have area for distributing it.
In the study, the researchers estimate that about 75 tons of animal urine and about 1 ton of wood ash are produced annually in Germany. The urine mass corresponds to about 1.1 tons of urea and 164.400 tons of phosphate. Wood ash can contain up to 18 % calcium.
Distinction from established slurry recycling techniques
Existing slurry treatment methods recover fertilizing substances—primarily nitrogen (as ammonia or ammonium salts) and phosphorus—using techniques like struvite precipitation, ammonia stripping, membrane processes, electrochemical processes, and biological nitrification/denitrification. However, many of these methods have not yet reached industrial scale. Most also require extensive technical equipment and are costly and energy-intensive, according to the Urinash developers.
The urinash process
The newly developed process that delivers green ammonia and calcium phosphate, has been realized in laboratory scale so far. As raw materials artificial urine (MPAU – multipurpose artificial urine) and ash obtained from a small nearby heat station combusting wood chips were used.
The urinash process involves these key steps:
- Converting urea in the urine to ammonia and CO2. This reaction is triggered by urease, which is either added directly or supplied by urease-containing bacteria
- Extracting calcium from the ash, where it exists as calcium carbonate, by leaching it with hydrochloric acid (HCl)
- Adding the resulting calcium chloride solution to the urea-depleted urine to produce calcium phosphate.
The residual materials from the various processes consist of small quantities of insoluble ash, minerals from the urine, biomass resulting from the biological conversion of urea, and a mineral-rich liquid derived from the ash treatment process.
Optimization of process steps
The primary goal was to maximize ammonia yield. However, the costs of chemicals were also to consider: commercial urease is expensive and scaling up the process by adding urease directly could cause economic and ecological challenges. Urease is usually produced from jack beans and scaling up its production is difficult.
The highest ammonia yield achieved during this study was 68 %, obtained by directly adding urease. In both cases – the enzymatic reaction and the biological-enzymatic reaction involving bacteria – the ammonia must subsequently be recovered from the reaction broth by distillation.
Calcium extraction from wood ash depends on the hydrochloric acid concentration. Starting with distilled water, which is already capable of leaching out small amounts of calcium, the amount of calcium recovered could be increased tenfold by adding 5M HCl.
What may be the next development steps for the urinash process?
So far, the researchers have been able to demonstrate the feasibility of the process on a laboratory scale and to estimate the potential annual yields of calcium phosphate and NH₃.
Further investigations on a larger scale, including the use of real animal urine, are required for a more accurate assessment. Among other things, the authors of the study point out that whilst the use of 5M HCl poses no problem at laboratory scale, it does present a challenge in an industrial-scale process. Here, the costs of chemicals, the necessary corrosion protection for the plant equipment and the management of waste generated could become problematic.
As well as looking into the feasibility of scaling up the process on an industrial scale, the researchers intend also to examine the framework conditions for its practical implementation. These include the collection, transport and processing of the two waste streams.
The laboratory study has recently been published in the journal Bioengineering.
Featured image: Matthias Böckel / Pixabay