Should we try to create a circular urine economy?

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

In excess, nitrogen and phosphorus in our waste streams can stimulate algal blooms and create dangerous conditions for 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, although it makes up only the ‘1 percent of waste water. Urine disposal could remove 75 percent of nitrogen and 55 percent of phosphorus from municipal wastewater treatment plants. And these nutrients could be recycled for use as fertilizer.

The friction is against systems that are used to the way things are. Wastewater infrastructures are put in place to take waste out of the house, without much thought, using pipes that already exist and toilets that people are used to. Diverting urine would require changing some of these details, while using the diverted material will require more acceptance of the waste as valuable.

The power of one

Abe Noe-Hays, co-founder of Rich Earth, said statistics about the place of urine in sewage 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 you sit on the toilet, pee naturally goes to the front of the bowl, while feces go to the back. Therefore, the front half of a split toilet collects urine and can only send it to a separate urine drain, while the back half remains connected to a sewage treatment system as usual. Separate tubes divert the urine to a collection tank. This system may not be perfect (a good target is a plus if used while standing and new plumbing is required), but it benefits from tweaking existing infrastructure.

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

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

If it can be separated, the urine can act to partially sterilize itself. The nitrogen in urine leaves the body as urea, a simple organic compound. Bacteria in pipes usually break down urea into ammonia. When the 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 in the body that might have gotten into the urine, Vinnerås said.

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

Ease of transportation

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. However, after applying the urine to the soil, 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 a fertilizer, and Noe-Hays said it works wonders from personal experience. Noe-Hays said there is no required concentration of nutrients for urine to be used as fertilizer. The mass of its components is what matters. Pouring 1,000 gallons of urine on one acre adds about 50 pounds of nitrogen. 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. “Hay doesn’t care if you’re applying the concentrate or the dilute,” he continued. “It just matters how many total pounds of fertilizer you get.”

For urine to be useful as a fertilizer for something more than a personal garden, it is useful to harness the ability to concentrate. A Rich Earth spinoff called Brightwater Tools is working to concentrate urine by freezing it, Noe-Hays said.

Freezing the water in the urine leaves the nutrients behind in a slurry that can be used on site or sent to a farm. Concentrating the urine makes the volume more manageable, especially if urine-diverting toilets are used in a commercial or office building. Instead of requiring several visits from urine-specific trucks to empty the tanks, the concentration hardware allows urine to be disinfected, pasteurized and frozen in situ. In tests, concentration levels reached a factor of 10, meaning trucks could come to collect every few months instead of every week.

Vinnerås came up with dehydration as another method to make urine fertilizers useful on a larger scale. Some of their research is looking at stopping the urea decomposition that occurs in pipes. If the urea does not break down, the nitrogen remains solid when dehydrated, creating a dry fertilizer of 15 to 20 percent nitrogen.

The advantage he sees with the production of a dry product is the opportunity to take over the existing infrastructure for the management of chemical fertilizers. Machinery for applying dry fertilizer already exists, and storing it can be as simple as stacking bags on top of each other.

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