On Gotland, Sweden's largest island, fresh water is scarce. At the same time, residents struggle with dangerous amounts of pollution from agriculture and sewage systems that cause harmful algae to breed in the surrounding Baltic Sea, Nature.com reports.

Starting in 2021, a team of researchers has begun working with a local company that rents portable toilets. The goal is to collect more than 70,000 litres of urine over 3 years from waterless urinals and specialised toilets in several locations during the summer tourist season. They dry the urine into concrete-like pieces, which they hammer into powder and press into fertiliser granules that fit into standard farm equipment. A local farmer uses the fertiliser to grow barley that will go to a brewery to make beer.

Researchers aim to take urine reuse "beyond the concept and put it into practice" on a large scale, says Prithvi Simha, a chemical process engineer at SLU and Sanitation360's chief technology officer. The aim is to provide a model that regions around the world could follow.

The Gotland project is part of a wave of similar efforts worldwide to separate urine from other wastewater and recycle it into products such as fertiliser. The practice, known as urine diversion, is being studied by groups of researchers in the United States, Australia, Switzerland, Ethiopia and South Africa, among others.

Scientists say urine diversion would have huge environmental and public health benefits if widely implemented worldwide. This is partly because urine is rich in nutrients that, instead of polluting bodies of water, could be used to fertilise crops or power industrial processes. According to Simha's estimates, humans produce enough urine to replace about a quarter of the world's current nitrogen and phosphorus fertilisers; it also contains potassium and many micronutrients. In addition, recycling urine could save large amounts of water and reduce some of the pressure on ageing and overloaded sewage systems.

Researchers and companies need to solve a range 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 serving entire buildings, with collection and maintenance services for the resulting concentrated or solidified product.

Urine diversion and reuse is the kind of "drastic reimagining of how we do human sanitation" that will become increasingly crucial as societies struggle with shortages of energy, water, and raw materials for agriculture and industry, believes biologist Lynn Broaddus, a sustainability consultant based in Minneapolis, Minn. "The truth is, it's valuable material."

Urine used to be a valuable commodity. In the past, some companies used it to fertilize crops, tan hides, wash clothes and make gunpowder. Then, in the late 19th and early 20th centuries, the modern model of centralised wastewater management emerged in England and spread around the world.

Under this model, flush toilets use water to quickly send urine, faeces and toilet paper down the drain, where it mixes with other fluids from households, industrial sources and sometimes stormwater runoff. In centralised sewage treatment plants, an energy-intensive process uses microbes to clean wastewater.

Depending on local regulations and the condition of a treatment plant, the wastewater discharged from the process may still contain high amounts of nitrogen and other nutrients, as well as other contaminants.

According to a study that modelled wastewater management systems in three US states and compared conventional wastewater management systems with hypothetical ones that divert urine and use recovered nutrients to replace synthetic fertilizers, depending on the technologies used, communities that used urine diversion could reduce total greenhouse gas emissions by up to 47%, energy use by up to 41%, freshwater use by about half, and nutrient pollution from wastewater by up to 64%.

However, the concept still remained niche, limited mainly to off-grid locations such as northern European ecological villages, rural dormitories and development projects in low-income environments. Urine diversion is just the first step in transforming the health system. The next step is figuring out what to do with it. In rural areas, people could store it in tanks to kill pathogens and then apply it to fields.

But urban environments are more difficult - and that's where most urine is produced. It's not practical to add a separate set of sewer pipes throughout the city to move urine to a central location. And because urine is about 95% water, it is too expensive to store and transport. So researchers are focusing on drying, concentrating or otherwise extracting nutrients from urine at the toilet or building level, leaving water behind.

Apart from water, most of it is urea, a nitrogen-rich compound that the body produces as a by-product of protein metabolism. Urea itself is useful: a synthetic version is a common nitrogen fertiliser. But it is also problematic: when combined with water, urea turns into ammonia gas, which contributes to the characteristic smell of urine. If not retained, ammonia pollutes the air and leaches valuable nitrogen.

The Gotland project team, led by environmental engineer Björn Vinnerås, has figured out how to dry urine into a solid urea mixed with other nutrients. The team is evaluating its latest prototype, a self-contained toilet that includes a built-in dryer, at the headquarters of the Swedish water and wastewater utility VA SYD in Malmö.

Given the progress made, mass production and automation of urine diversion technologies could be close at hand. And this would improve the speed of adoption of new methods of human waste management.