Recycling urine may be a way to boost plant growth | Science News for Students

Recycling urine may be a way to boost plant growth

New methods recover nutrients from human pee
Sep 8, 2017 — 7:00 am EST
urine reaction

Ammonia gas separates from urine in the tube on the right. It’s part of a new process that makes fertilizer from nutrients in the waste.

Aalto University/Lasse Lecklin

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

Every time you pee in a toilet, you flush away potentially valuable chemicals. One day, you might not have to.

Human urine is rich in nitrogen and phosphorus. Plants need these nutrients to grow. Now researchers in Finland have a new way to pull them out of human pee. And, they say, this process can turn a profit.  

Pee is mostly water. The rest of it are wastes that urine ferries out of the body. One of those wastes is excess nitrogen. Urine’s nitrogen exists mostly in the form of a chemical known as urea (Yu-REE-uh). Urine also removes excess phosphorus from the body.

Both nitrogen and phosphorus are nutrients that plants need to grow. But spraying urine directly onto farm fields is impractical. Untreated urine also can contain harmful germs that might make people sick. So researchers have been looking for ways to process pee into a safe, ready-to-apply plant fertilizer.

Surendra Pradhan is an environmental scientist at Aalto University in Finland. He got the inspiration for this innovation while doing research in Ghana. Large parts of that African country and other low-income nations have few systems for treating wastewater. Untreated human wastes can pollute bodies of water and lead to disease. Indeed, Pradhan notes, “Globally, wastewater is a very big issue.”

radish fertilizer
These radishes got a nitrogen boost from a fertilizer (white powdered crystals) created from human urine.
Surendra Pradhan

Methods already existed to make fertilizer from urine. One process produces crystals of magnesium-ammonium phosphate. Its common name is struvite. Growers could find it useful where they need to fertilize food crops with phosphorus and nitrogen. But farmers don’t always need both. Also, many farmers aren’t familiar with struvite, Pradhan says. So they may not buy it.

Instead, Pradhan was inspired to make a urine-sourced version of a product that is already commonly used. It's known as ammonium sulfate. His group described their innovative approach to making this nitrogen-rich product  in the May 2 issue of Environmental Science & Technology.

Two products at once

The first step in their new process adds calcium hydroxide to urine. Calcium hydroxide is an alkaline chemical, also known as a base. It increases the urine’s pH to above 12. (The alkaline range on the pH scale runs from just above 7 — which is neutral — to a high of 14.) The high pH kills any germs and sterilizes the urine, Pradhan says.  

Calcium hydroxide also reacts chemically with the urine. This pulls phosphorus out of the mixture in the form of calcium phosphate. That chemical can be sold as a phosphorus-rich fertilizer. The reaction also makes nitrogen-rich ammonia gas.

The new process diverts the gas into another chemical-reaction vessel. It contains sulfuric acid. Here, the ammonia reacts with the acid to make ammonium sulfate. That’s the common nitrogen-rich fertilizer.

The researchers will continue to work on these fertilizers to see whether they contain any potentially harmful contaminants. High levels of some metals, for instance, might pose health risks. But so far, Pradhan says, his team’s research suggests the risk of harmful contaminants is very, very low.

If all works out, the new urine-recycling process should be profitable. Number-crunching by Pradhan’s group shows that recycling 1,000 liters (264 gallons) of urine could yield a profit of about €2.25, or about $2.50. (€ is the symbol for euros, and here $ represents U.S. dollars.)

To be useful, the process must make money, Pradhan explains. Without a profit, people will likely not bother to recycle urine. And that will let a lot of valuable nutrients go to waste — literally.

Håkan Jönsson is an environmental engineer at the Swedish University of Agricultural Sciences in Uppsala. He did not work on Pradhan’s project. Using calcium hydroxide to do two things at once is “innovative,” he says.  And, he adds, “The total process seems to make economic sense.”

Still, Jönsson notes, the concentration of nitrogen in the fertilizer is only about 13 percent. That’s lower than the 21 percent found in commercial ammonium sulfate. In his view, then, there is still room to improve this process.

Collecting urine

To make money, Pradhan’s process needs a steady supply of urine that does not contain other bathroom wastes. The best way to do that is with a separating toilet. In this type, pee goes into one part. Feces go in another. Although still relatively rare, such toilets are becoming more common in Finland and Sweden.

Other researchers are also trying to turn human wastes into something useful. Two of Jönsson’s colleagues at the Swedish University of Agricultural Sciences are developing their own “No-Waste Toilet.” The name refers to its goal of turning human wastes into a useful fertilizer.

The Swedish team’s design uses wood ash to raise the pH of the urine. That step stops a chemical process that would have changed urine’s nitrogen-rich urea into ammonia. The wood ash stabilizes the nitrogen in urea. Now the urine can be dried without losing the nutrient value of its nitrogen. The resulting materials could be used as a fertilizer product, according to environmental engineers Jenna Senecal and Björn Vinnerås. They described the toilet in the May 15 issue of Science of the Total Environment.

Pradhan’s group and the Swedish team use different approaches. But their goals are the same — safe treatment and reuse of urine. Our bodies excrete excess nutrients from foods as wastes. Says Jönsson, “The recycling of the nutrients from our food is needed for a more sustainable society.”

Power Words

(more about Power Words)

acidic     An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

alkaline     An adjective that describes a chemical that produces hydroxide ions (OH-) in a solution. These solutions are also referred to as basic — as in the opposite of acidic — and have a pH above 7.

ammonia     A colorless gas with a nasty smell. Ammonia is a compound made from the elements nitrogen and hydrogen. It is used to make food and applied to farm fields as a fertilizer. Secreted by the kidneys, ammonia gives urine its characteristic odor. The chemical also occurs in the atmosphere and throughout the universe.

base     (in chemistry) A chemical that produces hydroxide ions (OH-) in a solution. Basic solutions are also referred to as alkaline.

calcium     A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

commercial     (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

concentration     (in chemistry) A measurement of how much of one substance has been dissolved into another.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

economics    (adj. economic) The social science that deals with the production, distribution and consumption of goods and services and with the theory and management of economies or economic systems. A person who studies economics is an economist.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

environmental engineer     A person who uses science to study and improve the natural environment.

environmental science     The study of ecosystems to help identify environmental problems and possible solutions. Environmental science can bring together many fields including physics, chemistry, biology and oceanography to understand how ecosystems function and how humans can coexist with them in harmony. People who work in this field are known as environmental scientists.

feces     A body's solid waste, made up of undigested food, bacteria and water. The feces of larger animals are sometimes also called dung.

fertilizer     Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

innovation    (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.

magnesium     A metallic element that is number 12 on the periodic table. It burns with a white light and is the eighth most abundant element in Earth’s crust.

nitrogen     A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn.

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

pH     A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

phosphate     A chemical containing one atom of phosphorus and four atoms of oxygen. It is a component of bones, hard white tooth enamel, and some minerals such as apatite.

phosphorus     A highly reactive, nonmetallic element occurring naturally in phosphates. Its scientific symbol is P. It is an important part of many chemicals and structures that are found in cells, such as membranes, and DNA.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

society     An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.

struvite      A natural phosphate-rich mineral found in kidney stones and bird guano. It has the chemical formula NH4MgPO4 (to which there are 6 molecules of water strongly attracted to it). In this formula, N stands for nitrogen, H for hydrogen, Mg for magnesium, P for phosphorus and O for oxygen. This mineral makes a good plant fertilizer.

sulfate     A family of chemical compounds that are related to sulfuric acid (H2SO4). Sulfates occur naturally in drinking water.

sulfuric acid     A strong acid having the chemical formula H2SO4. Used as a drain cleaner and in lead-acid car batteries, the liquid is able to burn tissues and eat through metals and even rock.

sustainable     An adjective to describe the use of resources in a such a way that they will continue to be available long into the future.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

urea     A nitrogen-rich chemical that the bodies of many animals produce after breaks down proteins, amino acids (the building blocks of proteins) or ammonia. People excrete excess nitrogen from the body — as urea — in urine. But many other mammals, amphibians and fish make urea as well. Synthetic urea is often a nitrogen source of plant fertilizers. In 1828, German chemist Friedrich Wöhler for the first time created urea in the laboratory. This discovery would lead to the widespread use of synthetic fertilizers in farming.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

wastewater     Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.


Journal: J. Senecal and B. Vinnerås. Urea stabilisation and concentration for urine-diverting dry toilets: Urine dehydration in ash. Science of The Total Environment. Vol. 586, May 15, 2017. doi: 10.1016/j.scitotenv.2017.02.038.

Journal: S.K. Pradhan et al. Nitrogen and phosphorus harvesting from human urine using a stripping, absorption, and precipitation process. Environmental Science & Technology. Vol. 51, May 2, 2017. doi: 10.1021/acs.est.6b05402.