University of Wisconsin-Madison engineers have created a new model that sheds light on why reverse osmosis systems recover some forms of ammonia from wastewater more effectively than others.
Mohan Qin, an assistant professor of civil and environmental engineering at UW-Madison and a corresponding author on the paper, says the research could have applications for agriculture in Wisconsin and beyond. She says the project grew out of visits to Wisconsin treatment plants where farms are already using membrane-based systems to separate water and nutrients from manure.
“We had the question of how ammonia moves in this treatment process,” Qin says. “After visiting a few treatment plants, we saw that this was a question we really wanted to answer.”
Water passes through thin-film semipermeable membranes during reverse osmosis. In the two-pass systems Qin studied, the first pass removed solids but was less effective at removing ammonia. The second pass adjusts the water’s pH value to more effectively separate ammonia.
However, reverse osmosis does not remove all forms of ammonia equally, and until now, the mechanism for why that’s the case has not been well understood. Qin collaborated with Ying Li, an associate professor of mechanical engineering at UW-Madison, to test why neutral ammonia and positively charged ammonium interact differently with reverse osmosis systems.
The study, published in the May 2026 issue of the journal Nature Communications, found that the neutral ammonia actually penetrates reverse osmosis membranes up to 30 times more easily than the positively charged ammonium does. They also found that the pH level in the water matters: near the neutral pH of 7, the reverse osmosis filter removed 89% of ammonia. And a pH below 8 ensures that more ammonia is present as ammonium, which is more easily stopped by the membranes.
Qin and Li’s team combined molecular modeling with lab-scale experiments. Li says the research shows why reverse osmosis membranes are generally better at stopping charged particles than neutral ones.
“Because it’s such a small scale, there has not been an effective experimental approach to monitor this process in situ,” Li says. “So molecular simulation becomes the most powerful way we can get to the sub-nanometer scale and quantify why charged molecules are rejected at a much higher rate than neutral molecules.”
Ammonia has supported global food supply and population growth for more than a century. It can be used directly as a fertilizer and is also a key component of a variety of nitrogen-based fertilizers. For decades, more than 80% of the ammonia produced in the United States has been used for fertilization, according to the U.S. Geological Survey.
Livestock manure, however, may account for as much as 60% of ammonia emissions in the United States. In manure systems, nitrogen can shift between ammonia gas and ammonium, depending on environmental conditions. Excess ammonia can contribute to environmental acidification and eutrophication, a process in which excess nutrients fuel algal blooms that overwhelm aquatic ecosystems. In people, atmospheric ammonia can cause short-term eye and lung irritation. Over the long term, fine particulate matter formed by ammonia can contribute to cardiovascular problems.
The reverse osmosis study builds on Qin’s ongoing research into resource recovery from wastewater and manure. The team tested its model against both lab-made samples and real manure wastewater samples. Qin says it could eventually help farmers recover more nutrients from waste while reducing ammonia emissions.
“Right now, manure is often stored in a lagoon where water evaporates over time, and the concentrated remains are used for fertilizer,” Qin says. “That process creates some problems, because it emits a lot of ammonia. Now we see more farms looking at advanced treatment processes to deal with that, and reverse osmosis is one of the most effective available methods to treat manure.”
The National Science Foundation’s Division of Chemical, Bioengineering, Environmental and Transport Systems and the National Alliance for Water Innovation, through the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy and Industrial Technologies Office, supported the research.
Featured image caption: Civil and Environmental Engineering Assistant Professor Mohan Qin, left, discusses ammonia filtration with Ying Li, an associate professor of mechanical engineering. Qin and Li collaborated on research into why reverse osmosis systems recover different forms of ammonia at different rates. Photo by Joel Hallberg.