Wastewater can replace clean water as a source for hydrogen production, eliminating a major drawback to hydrogen fuel and reducing water treatment costs by up to 47%, according to new research from Princeton Engineering.
The findings, reported Sept. 24 in the journal Water Research, are a step toward making hydrogen a practical pathway to decarbonize industries that are difficult to electrify, such as steel and fertilizer production.
Z. Jason Ren , the senior study author, said that current electrolytic hydrogen production requires a large amount of clean water, increasing costs and straining local water supplies. His research team wanted to find out whether treated water processed by wastewater plants could be substituted.
"Hydrogen infrastructure generally competes with local fresh water use," said Ren, a professor of civil and environmental engineering and the Andlinger Center for Energy and the Environment . "But every town has a wastewater treatment plant, and that's a very distributed source of water for the hydrogen economy."
Producing hydrogen with renewable energy, known as green hydrogen, relies on electrolysis to split water into hydrogen and oxygen gas. The water flows into an electrolyzer, where electric current causes positively charged hydrogen ions (protons) to move from an anode across a specialized membrane to a cathode, where the protons combine with electrons to form hydrogen gas.
Most of the hydrogen currently produced in the United States is known as blue hydrogen, in which natural gas provides the energy for the process and at least some of the carbon dioxide produced is captured and stored underground — making blue hydrogen a lower-carbon energy source than using natural gas directly.
Green hydrogen electrolysis uses renewable electricity and produces much lower carbon emissions. But it typically requires ultrapure water, created by treating tap water or groundwater with processes such as reverse osmosis to remove impurities that could interfere with electrolysis.
The Princeton team tested using treated wastewater rather than tap water to bypass the purification process. In this scenario, the wastewater is "reclaimed," meaning treated to the point where it could be discharged to aquifers or used for irrigation or industrial cooling.
The method had been tried before but typically failed after a limited time, Ren said. To uncover the reason for the failure, Lin Du, a Ph.D. student in Ren's lab, performed carefully designed diagnostic experiments in a proton exchange membrane water electrolyzer — the same technology currently used commercially with ultrapure water.
Du and his co-authors used a combination of electrochemical tests and advanced microscopic imaging to compare the performance of pure water to reclaimed wastewater in the reactors. They observed that system performance declined rapidly with reclaimed water, while the same setup with pure water continued to function stably. Through experimental and modeling analyses, they identified ions of calcium and magnesium — the same minerals that cause scale buildup on household faucets and kettles — as the main cause of performance loss. These ions stick to the membrane, blocking ion transport and turning it from a porous pathway to a solid barrier.
To address this issue, the researchers came up with a simple solution: acidifying the water with sulfuric acid. The resulting acidic buffer acts as a rich source of protons that outcompete other ions, maintaining ion conductivity, sustaining electrical current and enabling continuous hydrogen production.
"It's expensive to remove all those ions so you have ultrapure water going into the electrolyzer," said Ren. "Now, you can just acidify it a bit, you put ion-containing water into the electrolyzer, and it lasts for more than 300 hours without apparent issues."
His team estimated that using reclaimed wastewater rather than purified water could reduce the cost of treating water for hydrogen production by about 47% and the energy cost of that treatment by about 62%.
Crucially, said Ren, "this acid is recirculated, so it's never getting out of the system," which is important from both an environmental and cost perspective. Likewise, the calcium and magnesium ions remain in soluble phase without interfering with the circulation.
Ren and his team are working with industry partners to test how their approach functions on a larger scale, as well as how it might work with pretreated seawater as an input. Last year, they published a study on optimizing both water and cost savings in hydrogen production, identifying the best spots in the United States to collocate hydrogen facilities with wastewater facilities where reclaimed water is abundant. That work was led by Jinyue Jerry Jiang, who was a Maeder Graduate Fellow at the Andlinger Center and completed his Ph.D. at Princeton earlier this year.
"We wanted to really look into the possibility of using reclaimed water to enable a national hydrogen strategy," said Ren. "We do both deep technical research and big-picture analytical work to serve both scientific needs and industry needs."
The paper, " Electrolytic hydrogen production from acidified wastewater effluent ," was published Sept. 24 in Water Research. In addition to Du, Jiang and Ren, co-authors included Guangye Zhou, Yuqing Yan and Ryan Kingsbury of Princeton; and Anthony Ku of Xiron Global Ltd., a non-resident fellow at the Andlinger Center. Support for this work was provided by New Jersey Resources and the Andlinger Center for Energy and the Environment.
