Sewage is not something most people want to give a second thought to, but it contains a trove of valuable nutrients. One of those is ammonia, a key ingredient of fertilizer.
Now, researchers report a way to use sunlight to recover ammonia from wastewater. The cheap, efficient process is a practical way to reuse the nutrient on farms and keep it from reaching the environment, where it can cause harm. The work appears in the journal Nature Sustainability.
Ammonia is a source of nitrogen in fertilizers. Around 240 million tons of ammonia are produced every year globally using the Haber-Bosch process. The method, while critical for feeding the world, takes huge amounts of energy and has a large carbon footprint.
Meanwhile, two-thirds of the fertilizer farmers apply to their fields escapes into the ground as run-off. The excessive nitrogen reaches water bodies where it can harm aquatic life and lead to toxic algae blooms.
Removing ammonia from wastewater is doable but the process is expensive so treatment plants mostly decompose ammonia into less harmful compounds. So researchers have been devising various techniques and developing novel materials to capture ammonia and other nutrients from wastewater.
Ning Xu and colleagues at Nanjing University came up with an efficient solar-driven method. They made a solar still—a container with a clear plastic or glass top—that uses the sun to purify water. The sun’s heat evaporates water in the sill, and the clean vapors are condensed and collected.
Because fertilizer runoff and industrial wastewater mostly contain ammonia in the form of ammonium, the team devised a way to convert the ammonium into ammonia. They coated a plastic sponge with a thin layer of a black, heat-absorbing substance called titanium carbide. Then they attached chemical groups called amino groups to the surface of the sponge.
Floating on the wastewater enclosed in the solar still, the black sponge absorbed heat while its amino groups converted the ammonium in wastewater into ammonia. The heat evaporated both the ammonia and water, which were condensed and captured for use.
Focusing sunlight on the sponge cleans it for reuse while producing another useful commodity: hydrochloric acid. The researchers analyzed the economics of the method in 24 different regions of China. Taking into account the cost of the specialty sponge materials, they found that it had “excellent economics benefit and revenue” generating a profit of $90 per square meter of the sponge, and a payback period of about 3.5 years.
Source: Qi Zhang et al. Solar-driven efficient and selective ammonia recovery from ammonium-containing wastewater. Nature Sustainability, 2025.
Image: ©Anthropocene Magazine
