The urine revolution: how recycling pee could help to save the world

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On Gotland, the largest island in Sweden, fresh water is scarce. At the same time, residents are battling dangerous amounts of pollution from agriculture and sewer systems that causes harmful algal blooms in the surrounding Baltic Sea. These can kill fish and make people ill.

To help solve this set of environmental challenges, the island is pinning its hopes on a single, unlikely substance that connects them: human urine.

Starting in 2021, a team of researchers began collaborating with a local company that rents out portable toilets. The goal is to collect more than 70,000 litres of urine over 3 years from waterless urinals and specialized toilets at several locations during the booming summer tourist season. The team is from the Swedish University of Agricultural Sciences (SLU) in Uppsala, which has spun off a company called Sanitation360. Using a process that the researchers developed, they are drying the urine into concrete-like chunks that they hammer into a powder and press into fertilizer pellets that fit into standard farming equipment. A local farmer uses the fertilizer to grow barley that will go to a brewery to make ale — which, after consumption, could enter the cycle all over again.

The researchers aim to take urine reuse “beyond concept and into practice” on a large scale, says Prithvi Simha, a chemical-process engineer at the SLU and Sanitation360’s chief technology officer. The aim is to provide a model that regions around the world could follow. “The ambition is that everyone, everywhere, does this practice.”

The Gotland project is part of a wave of similar efforts worldwide to separate urine from the rest of sewage and to recycle it into products such as fertilizer. That practice, known as urine diversion, is being studied by groups in the United States, Australia, Switzerland, Ethiopia and South Africa, among other places. The efforts reach far beyond the confines of university labs. Waterless urinals connect to basement treatment systems in offices in Oregon and the Netherlands. In Paris, there are plans to install urine-diverting toilets in a 1,000-resident eco-quarter being built in the 14th district of the city. The European Space Agency is to put 80 urine-diverting toilets into its Paris headquarters, which will begin operating later this year. According to proponents of urine diversion, it could see uses in sites from temporary military outposts to refugee encampments, rich urban centres and sprawling slums.

Scientists say that urine diversion would have huge environmental and public-health benefits if deployed on a large scale around the world. That’s in part because urine is rich in nutrients that, instead of polluting water bodies, could go towards fertilizing crops or feed into industrial processes. According to Simha’s estimates, humans produce enough urine to replace about one-quarter of current nitrogen and phosphorus fertilizers worldwide; it also contains potassium and many micronutrients (see ‘What’s in urine’). On top of that, not flushing urine down the drain could save vast amounts of water and reduce some of the strain on ageing and overloaded sewer systems.

Thanks to advances in toilets and urine-treatment strategies, many components of urine diversion could soon be ready for widespread roll-out, according to experts in the field. But there are also big obstacles to radically re-engineering one of the most basic aspects of life. Researchers and companies need to solve a number of problems, from improving the design of urine-diverting toilets to making it easier to treat urine and turn it into valuable products. This could involve chemical-treatment systems connected to individual toilets or basement devices that serve entire buildings, with pick-up and maintenance services for the resulting concentrated or solidified product (see ‘From pee to products’). Then there are broader questions of social change and acceptance, related both to varying levels of cultural taboos around human waste and to deeply entrenched conventions about industrial sewage and food systems.

Urine diversion and reuse is the type of “drastic reimagining of how we do human sanitation” that will become increasingly crucial as societies battle shortages in energy, water and raw materials for agriculture and industry, says biologist Lynn Broaddus, a sustainability consultant in Minneapolis, Minnesota, who is former president of the Water Environment Federation in Alexandria, Virginia, an association of water-quality professionals worldwide. “The fact of the matter is, it’s valuable stuff.”

Mixed waste

Urine used to be a valuable commodity. In the past, some societies used it for fertilizing crops, tanning leather, washing clothes and producing gunpowder. Then, in the late nineteenth and early twentieth century, the modern model of centralized sewage management arose in England and spread worldwide, ultimately leading to what has been called urine blindness.

In this model, flush toilets use water to quickly send urine, faeces and toilet paper into sewers, where it mixes with other liquids from households, industrial sources and sometimes storm run-off. At centralized treatment plants, an energy-intensive process uses microbes to clean the sewage.

Depending on local regulations and a treatment plant’s condition, the wastewater discharged from the process can still contain a lot of nitrogen and other nutrients, as well as some other contaminants. And 57% of the world’s population isn’t connected to centralized sewer systems at all (see ‘Human sewage’).

Scientists are working on ways to make centralized systems more sustainable and less polluting, but, beginning in Sweden in the 1990s, some researchers began pushing for more fundamental change. The end-of-pipe advances are “just, you know, another evolution of the same damn thing”, says Nancy Love, an environmental engineer at the University of Michigan in Ann Arbor. Urine diversion would be “transformative”, she says. In a study1 that modelled wastewater-management systems in three US states, she and her colleagues compared conventional wastewater systems with hypothetical ones that divert urine and use the recovered nutrients to replace synthetic fertilizers. They projected that communities with urine diversion could lower their overall greenhouse-gas emissions by up to 47%, energy consumption by up to 41%, freshwater use by about half, and nutrient pollution from the wastewater by up to 64%, depending on the technologies used.

Still, the concept has remained niche, mostly limited to off-grid locales such as northern European eco-villages, rural outhouses and development projects in low-income settings.

A lot of the lag is a result of the toilets themselves, says Tove Larsen, a chemical engineer at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) in Dübendorf. First sold in the 1990s and 2000s, most urine-diverting toilets have a small basin at the front to capture the liquid — a set-up that requires careful aim. Other designs have incorporated foot-powered conveyor belts that let urine drain away while transporting the faeces to a composting vault, or sensors that operate valves to direct the urine to separate outlets….”

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