Urine sample. Credit: Turbotorque
Engineers have designed a system that uses electricity and waste heat generated by solar panels to extract nitrogen fertiliser from human urine, leaving behind clean wastewater which is safer to discharge or reuse for irrigation.
The proof-of-concept photovoltaic–thermal electrochemical stripping (ECS) system, known as solar-ECS, has been published in a paper in Nature Water.
“This project is about turning a waste problem into a resource opportunity,” says study senior author William Tarpeh, an assistant professor of chemical engineering at Standford University in the US.
“With this system, we’re capturing nutrients that would otherwise be flushed away or cause environmental damage and turning them into something valuable – fertiliser for crops – and doing it without needing access to a power grid.”
Orisa Coombs, the study’s lead author and PhD student in mechanical engineering at Stanford, adds: “Each person produces enough nitrogen in their urine to fertilise a garden, but much of the world is reliant on expensive imported fertilisers instead.”
The demand for nitrogen fertiliser is currently being met by the energy- and carbon-intensive Haber–Bosch process which produces 150 megatonnes of ammonia each year and is set to increase by 2.3% per year.
But the facilities in which this process is carried out are disproportionately located in high-income countries and the fertiliser’s subsequent use on cropland is similarly unbalanced.
“In over fertilised regions, 50–70% of the applied nitrogen is lost to the environment, exacerbating aqueous ammonia pollution and emitting nitrous oxide, while
in under fertilised regions, such as sub-Saharan Africa, low crop yields contribute to food insecurity in up to 70% of the population,” the authors write.
The nitrogen present in the combined urine of humans around the world contains enough nitrogen to meet 14.4% of this global demand.
Capturing nitrogen from urine would prevent its discharge into bodies of water, where excess nutrients from fertiliser runoff and sewage contributes to the increased growth of algae and phytoplankton which deplete oxygen in the water (eutrophication).
“The decentralised nature of urine production presents opportunities for distributed fertiliser production, a circular nitrogen economy and improved sanitation,” the authors write. They add that “producing fertiliser near the point of use can reduce transportation costs and expand access in regions reliant on expensive imported fertilisers”.
While ECS has been already been used to recover ammonium sulphate fertilizer from urine and other wastewaters, the authors set out to identify how to optimise its operating conditions to “maximise energy efficiency and economic viability for distributed operation”.
Their prototype employs a series of chambers separated by membranes to remove ammonia from urine.
Electricity generated by solar voltaic cells converts ammonia to ammonium ions at the device’s cathode. These ions migrate through a membrane to the anode, where they are converted into ammonium ions. They then migrate through a second membrane to the trap chamber where they react with sulphuric acid to form ammonium sulphate – a common fertiliser.
Crucially, the ECS reactor can be heated up to 60°C by waste heat produced by the photovoltaic solar panel it is attached to, which speeds up this process.
“Over 80% of the sunlight incident on solar panels becomes waste heat, which reduces the light-to-electricity conversion efficiency by up to 0.65% per degree Celsius increase in photovoltaic (PV) cell temperature,” the authors explain.
The combined system has the doubly beneficial effect of cooling the solar panel, therefore increasing electricity production, while heating the ECS reactor for increased nitrogen recovery.
“You don’t need a giant chemical plant or even a wall socket. With enough sunshine, you can produce fertiliser right where it’s needed, and potentially even store or sell excess electricity,” says Coombs.
The team developed a model to predict how sunlight, temperature, and electrical configuration can affect the system’s performance and economics in 3 different regions: Palo Alto, California and Oklahoma City, Oklahoma in the US, and Kampala, Uganda.
They estimate the system could generate revenues of up to US$2.18 per kg of nitrogen recovered in the US, and more than double that in Uganda ($4.13).
“We often think of water, food, and energy as completely separate systems, but this is one of those rare cases where engineering innovation can help solve multiple problems at once,” says Coombs.
“It’s clean, it’s scalable, and it’s literally powered by the sun.”