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Removing urine from wastewater and using it as fertilizer has the potential to decrease nutrient loading in water bodies and boost sustainability by making use of a common waste material.

In excess, nitrogen and phosphorus in our waste streams can stimulate algal blooms and create conditions dangerous to marine and lake ecosystems and human health. According to the website of the Rich Earth Institute, a Vermont-based company focused on using human waste as a resource, most of the nitrogen and phosphorus in wastewater comes from human urine, even though it makes up only 1 percent of wastewater. Removing urine could remove 75 percent of the nitrogen and 55 percent of the phosphorus [richearthinstitute.org] from municipal wastewater treatment plants. And those nutrients could then be recycled for use as fertilizer.

The rub is against systems that are used to the way things are. Wastewater infrastructure is set up to get waste out of the house, without much thought, using pipes that already exist and toilets people are used to. Urine diversion would require changing some of these details, while putting the diverted material to use will need more acceptance of waste as valuable.

Abe Noe-Hays, co-founder of Rich Earth, said that the statistics on urine’s place in wastewater is what got the ball rolling on urine diversion, an attempt to keep it out of the waste stream in the first place.

A urine diverting toilet makes use of the body’s anatomy. When sitting on the toilet, pee naturally goes toward the front of the bowl, while feces go to the back. Therefore, the front half of a divided toilet bowl catches the urine and can send it to a separate drain for urine only, while the back remains connected to a wastewater treatment system per usual. Separate pipes divert the urine to a collection tank. This system might not be perfect—good aim is a bonus if it's used while standing, and some new plumbing is required—but it does benefit from tweaking existing infrastructure.

If there’s any possibility of fecal mixing, the World Health Organization has (believe it or not) guidelines for how long urine should be stored before being used as fertilizer. After six months at room temperature, urine has self-sanitized enough to be used on anything, including raw produce, Noe-Hays said.

The key here is that if the urine is urine only, it’s ready to go as a nitrogen- and phosphorus-rich fertilizer the moment it leaves the body. But getting a good separation is important. Feces are the main source of pathogens in the collected urine, according to Björn Vinnerås, environmental engineering professor at the Swedish University of Agricultural Sciences. Urine-diverting toilets aren’t perfect, he said, and some mixing is unavoidable.

If it can be separated, urine can act to partly sterilize itself. The nitrogen in urine leaves the body as urea, a simple organic compound. Bacteria in pipes typically break down urea into ammonia. When urine is sitting in a container, the ammonia raises the pH of the solution to about eight or nine. The high pH environment kills any pathogens from the body that might have entered the urine, Vinnerås said.

“It’s like a Twinkie,” Noe-Hays said, referring to urine’s long shelf-life.

Noe-Hays was part of a study that looked at concentrations of pharmaceuticals in urine. Caffeine and ibuprofen were among the most common and abundant. After urine was applied to soil, however, the drug concentration in the crops was extremely low. According to the study, to consume the amount of caffeine in a cup of coffee from urine-fertilized produce, a person would have to eat a pound of the produce every day for about 2,000 years, Noe-Hays said.

Gardeners often use urine as fertilizer, and Noe-Hays said it works wonders from his personal experience. Noe-Hays said there is no necessary concentration of nutrients for urine to be used as fertilizer. The mass of its components is what matters. If pouring 1,000 gallons of urine on an acre, there are about 50 pounds of nitrogen added. Using a concentrate 10 times stronger than diluted urine, only 100 gallons would need to be applied to get the same impact, Noe-Hays said. “The hay doesn’t care whether you’re applying the concentrate or the dilute,” he continued. “It just matters how many total pounds of fertilizer it gets.”

For urine to be useful as fertilizer for something more than a personal garden, it’s helpful to take advantage of the ability to concentrate it. A spinoff of Rich Earth called Brightwater Tools is working on concentrating urine by freezing it, Noe-Hays said.

Freezing the water out of urine leaves behind the nutrients in a slurry that can be used onsite or shipped to a farm. Concentrating the urine makes the volume more manageable, particularly if urine-diverting toilets are used in a commercial or office building. Instead of needing multiple visits from urine-specific trucks to empty the tanks, the concentration hardware allows the urine to be sanitized, pasteurized, and freeze-concentrated on-site. In trials, the concentration levels reached a factor of 10, meaning trucks might come to collect every few months instead of every week.

Vinnerås brought up dehydration as another method for making urine fertilizer useful on a larger scale. Some of his research is looking at stopping the urea breakdown that happens in pipes. If urea doesn’t break down, the nitrogen remains solid when dehydrated, creating a dry fertilizer of about 15 to 20 percent nitrogen.

The advantage he sees with producing a dry product is the chance to piggyback on existing infrastructure for chemical fertilizer management. Machinery already exists to apply dry fertilizer, and storing it can be as simple as piling bags on top of each other.

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