Can we use waste water to make fertilizer?

Nitrogen fertilizers play an important role in global crop production. About half of the human population is fed by food grown with fertilizers. Although the planet’s atmosphere is about 78 percent nitrogen, it doesn’t come in a reactive form for plants to use. It wasn’t until 1908 that chemists developed a technique to convert nitrogen from the atmosphere into a state of synthetic nitrogen that plants could use.

This technique, called the Haber-Bosch process, is how nitrogen is captured from the air and reacts with hydrogen to produce ammonia, an effective fertilizer that plants can absorb from the soil. This process is standard industrial procedure for making ammonia today, but it is responsible for about 1.4 percent of global carbon dioxide emissions.

“The hydrogen in ammonia comes from fossil fuels, such as natural gas, and nitrogen comes from air,” said Saurajyoti Kar, a postdoctoral researcher at Argonne National Laboratory. “The use of fossil fuel as a feedstock and energy source for the conversion process increases the energy and environmental burden of producing the nitrogen-rich fertilizer using [the] conventional production process.”

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The global ammonia market is expected to reach $82.40 billion by 2026. Given the energy intensity of the Haber-Bosch process, producers must adopt greener approaches to meet the increasing demand for fertilizers. In a recent Science of the total environment study, researchers evaluated the process of removing ammonia from wastewater and converting it into fertilizer, which may be a more sustainable alternative.

Municipal wastewater generally contains a high concentration of nitrogen and phosphorus, says Kar, who was involved in the study. At treatment plants, the wastewater is treated to reduce this concentration and prevent problems, such as eutrophication, which can lead to algae overgrowth, when discharged into surface water bodies, she adds.

By capturing nitrogen from wastewater, producers can avoid the energy-intensive production of ammonia. In addition, it reuses nitrogen already fixed in the atmosphere. “One of the ways to capture the nitrogen in wastewater treatment plants is by air stripping,” says Kar. “At a certain process temperature, excess ammonia from the wastewater stream transitions from the liquid to the gas phase, which can further react with acids to form stable nitrogen-rich fertilizers.”

The authors conducted a life cycle analysis and found that air stripping ammonia from wastewater treatment plants to make nitrogen-rich fertilizer produces six times less greenhouse gas emissions than the Haber-Bosch process. Air-stripping technology produces between 0.2 and 0.5 kilograms of carbon dioxide equivalent per kilogram of ammonium sulfate, which is significantly lower than the 2.5 kilograms of carbon dioxide equivalent per kilogram of ammonium sulfate of the Haber-Bosch process. Using renewable energy sources for the venting process can further reduce emissions.

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“The use of air stripe-based nitrogen fertilizers can reduce greenhouse gas emissions from agriculture and contribute to agriculture decarbonisation goals,” says Kar. The agricultural sector accounted for 11 percent of the country’s greenhouse gas emissions in 2020, including fertilizer application.

Aside from environmental benefits, wastewater treatment plants can also have economic benefits. If they set up the infrastructure for an air stripping system, the capital and operating costs could be outweighed by the revenue from selling the recovered ammonia, says Kar.

All in all, the research shows that there is a more sustainable alternative to the energy-intensive nitrogen production process. While air stripping can produce fertilizer on a smaller scale than the standard Haber-Bosch process, recovering and reusing any amount of nitrogen still helps minimize greenhouse gas emissions and prevent pollutants from reaching water sources.

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