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Technique may sustainably recover nutrients from urine

Membrane-based technique and closed-loop system may recover ammonia as valuable fertilizer product while removing it as pollutant from waste streams.

Ammonia is a key component of fertilizer and vital in supporting plant growth, ultimately providing food for populations around the world. It is also a major pollutant that, when released into the environment, could pollute aquatic settings and damage ecosystems, triggering destructive algal blooms, dead zones and fish kills, according to the Columbia University School of Engineering & Applied Science.

To date, most ammonia capture is done through an extremely energy-intensive technique known as the Haber-Bosch process, which is used by industry across the globe to produce fertilizer and accounts for 1-2% of the world’s annual energy consumption, the announcement said.

A Columbia Engineering team led by Ngai Yin Yip, assistant professor of earth and environmental engineering, reported the recovery of ammonia through a new method that uses a very low level of energy -- approximately a fifth of the energy used by the Haber-Bosch process. In addition, because the technique recycles ammonia in a closed loop, the ammonia can be recaptured for reuse in fertilizer and other industrial products. The findings were published by ACS Sustainable Chemistry & Engineering.

The management of nitrogen, an essential nutrient for life, has been recognized by the National Academy of Engineering as one of the Grand Challenges of the 21st century. Yip’s group, which focuses on advancing sustainable production of both energy and water, wanted to invent a better, more ecological way to produce nitrogen, of which ammonia is a bioavailable form.

“It was clear that we needed a paradigm shift to transition to a circular economy model, where nitrogen is recovered and recycled, instead of the current unsustainable linear approach of costly production, utilization and then discarding pollutants to the environment,” Yip said.

Yip’s team has expertise in membrane distillation, a technique that drives the permeation of volatile species, in this case, ammonia, from an input stream to a collector stream, while the non-volatile species remain in the input stream, the announcement explained. The volatile species are driven across the membrane by a difference in vapor pressure, which is dependent on temperature and concentration.

The researchers developed a technique they call “isothermal membrane distillation with acidic collector” (IMD-AC) that uses low-temperature heat and applied it to selectively separate and capture ammonia from the ammonia-rich waste stream of urine (simulated for this project).

“Because our process is driven by moderate temperatures as low as 20-60°C, the energy can be supplied by cheap or even free waste heat from, for instance, cooling tower water, bath water or solar thermal collectors,” Yip said.

Next steps for the team include exploring ways to recover phosphorus -- another key ingredient of fertilizer -- sustainably and cheaply from urine.

“Now that we’ve demonstrated the sustainable recovery of nitrogen from urine, we think that the growing population and sanitation trends present ideal opportunities for the introduction of decentralized urine diversion facilities for nutrient recovery without costly retrofits or overhauls of the existing system, shifting wastewater management to a more sustainable and efficient paradigm,” Yip added.

While the research was focused on urban municipal waste streams, the concept could also be applied to livestock manure management systems, especially liquid or slurry systems.

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