Almost half of the planet's population depends on synthetic fertilizers to grow the food they eat. But that fertilizer comes at a cost - about two percent of the world's energy budget. Improving efficiency and cutting costs of producing fertilizer would have big, global impacts.
To that end, researchers at Lawrence Livermore National Laboratory (LLNL) are developing a process to convert wastewater into clean water and recycled fertilizer.
By producing fertilizer domestically, the method could contribute to ensuring food security in the U.S. Much of the fertilizer supply chain is currently located outside of the country, which can have stark consequences. For example, during the 2022 invasion of Ukraine, costs of fertilizer in the U.S. quadrupled due to the impact in supply.
"That can completely destroy a farm's viability and revenue," said LLNL scientist Jeremy Feaster. "Fertilizer production plays a really big role for us in terms of how much food we're able to grow to support the nation."
Feaster was inspired to pursue this line of research after speaking with farmers and growers in California. The runoff from fertilized crops produces wastewater that is full of nitrates and unsafe to consume. Farmers wanted a way to clean up that water. But Feaster realized that LLNL might be able to do one better - clean the water and recover and transform the nitrates back into fertilizer.
To tackle this idea, Feaster paired up with LLNL scientist Steven Hawks. The first stage of the process involves pulling out the nitrates, ions that are common components in fertilizer and explosives.
That task isn't easy, though. Desalinating water, or removing salts and ions, typically requires a lot of energy.
"Ions are small, and they are really happy to be dissolved and surrounded by water," said Hawks. "They don't want to come back out. They don't want to form salt again."
Hawks took on this challenge using a technology that resembles electrodialysis. The method applies a voltage to drive ions through ion-exchange membranes, separating charged species into different channels while leaving neutral water molecules behind.
"If you imagine the device, it's built like a sandwich of alternating membranes and flow channels. When you apply a voltage, the cations slip through the cation-exchange membranes, and the anions slip through the anion-exchange membranes. Because the membranes are arranged in sequence, the ions collect in every other channel, leaving the in-between channels with desalinated water," said Hawks. "So, at the end, you get two separate streams coming out - one that's saltier, and one that's clean.
"Typically, we throw away or get rid of the salty water as cheaply as possible. But in our case, it's value added, and its value scales with its concentration."
That's where Feaster and his team come back into the picture. They take the brine and put it into a 3D-printed, customized chemical reactor. The scientists are working to fine-tune the device to ensure that every nitrate molecule interacts with the catalyst and therefore transforms into useable fertilizer.
"A lot of farmers have to pay people to deal with this concentrated waste. The nice thing is that we're giving them something of value back," said Feaster. "We can actually take what we would consider a waste, run it in our system, and turn it back into a few different types of fertilizers - things that they would actually be able to use on crops."
LLNL is uniquely suited for this research because of the Laboratory's Site 300. Located 15 miles east of LLNL's main site, Site 300 is home to world-class facilities to support the development and testing of explosive materials. LLNL and the Department of Energy are working to clean up legacy contamination at the site that includes nitrates in groundwater, providing a source of contaminated water for Feaster and Hawks to test their technology.
Feaster and Hawks hope to scale and test their technology at Site 300 in the coming years.
"The hope is for the environmental remediation effort at Site 300 to integrate our process into one of their existing systems, for us to treat nitrate-contaminated groundwater in real time and become part of the remediation process," said Feaster. "That will allow us to demonstrate that it's working at scale and contribute value to the Laboratory's mission. Then, we can figure out how to get it into the hands of farmers."
