ANN ARBOR – Recycling urine by diverting it away from municipal wastewater plants and processing the nutrient-rich liquid into crop fertilizer would have several environmental benefits when used at city scale, according to a new study led by the University of Michigan.
Published on Dec. 15 in the online Environmental Science & Technology journal, the study compared the expected environmental impacts of large-scale, streamlined urine-diversion and fertilizer-processing systems -- which currently don’t exist -- to conventional fertilizer production and wastewater treatment methods.
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The researchers found that recycling and diverting urine led to significant reductions in energy use, greenhouse gas emissions, freshwater consumptions and instances of fueling algal blooms in bodies of water.
Depending on the impact category, the reductions ranged from 26% to 64%.
“Urine diversion consistently had lower environmental impacts than conventional systems,” lead author Stephen Hilton, who conducted the study for his master’s thesis at U-M’s School for Environment and Sustainability, said in a statement.
“Our analyses clearly indicate that the well-defined benefits -- reduced wastewater management requirements and avoided synthetic fertilizer production -- exceed the environmental impacts of urine collection, processing and transport, suggesting that further efforts to develop such systems are warranted.”
Essential nutrients phosphorus, nitrogen and potassium are found in urine and have been used for thousands of years as crop fertilizers.
Recently, studies have been focusing on urine recycling as a way to produce renewable fertilizers while decreasing the amount of chemicals and energy needed for wastewater treatment.
While no urine recycling and diversion systems exist on a city scale, several projects are underway -- including one at U-M -- that demonstrate small-scale impact. Hilton used data from U-M’s project, as well as a project led by the Rich Earth Institute in Vermont to model the impact city-scale urine diversion and recycling would have on the environment.
Hilton also studied wastewater treatment using data from treatment plants in Michigan, Vermont and Virginia.
The one downside to processing urine into fertilizer was that one method led to an increase in acidification.
“This is the first in-depth analysis of the environmental performance and benefits of large-scale urine recycling relative to conventional wastewater treatment and fertilizer production,” Greg Keoleian, senior author of the ES&T paper and director of the Center for Sustainable Systems at the U-M School for Environment and Sustainability said in a statement. Keoleian chaired Hilton’s thesis committee.
Roughly half of the world’s food supply relies on synthetic fertilizers made from nonrenewable resources, with the production of nitrogen fertilizer being responsible for 1.2% of the globe’s energy use and related greenhouse gas emissions.
Additionally, water and waste water systems take up 2% of all electricity in the United States, and one of the most energy-intensive processes is nutrient removal.
Although urine diversion and recycling was shown to have several environmental benefits, negative impacts it would have include the collection and transportation of urine, processing it into fertilizer and shipping the end product around the country.