From pee to tree fertiliser

Pee is mostly water, with the rest of it wastes expelled from the body. One of those wastes is excess phosphorus and another is excess nitrogen, which exists mostly in the form of a chemical known as urea.

Both nitrogen and phosphorus are nutrients that plants need to grow, but spraying urine directly onto farm fields is impractical and unhealthy.  So researchers have been looking for ways to process pee into a safe plant fertiliser that can be used widely.

While some methods already exist to make fertiliser from urine, including producing crystals of magnesium-ammonium phosphate (struvite), growers aren’t always familiar with struvite and don’t necessarily need both phosphorus and nitrogen.

Instead, Dr Surendra Pradhan, an environmental scientist at Aalto University in Finland, was inspired to make a urine-sourced version of a product that is already commonly used, known as ammonium sulfate. 

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, as Dr Pradhan told Science News for Students, “globally, wastewater is a very big issue”.

The science 

The first step in their new process adds calcium hydroxide to urine. This increases the pH, killing off any germs and sterilising the urine.

Calcium hydroxide also reacts chemically with the urine to pull the phosphorus out in the form of calcium phosphate, which can be sold as a phosphorus-rich fertiliser.

The reaction also makes nitrogen-rich ammonia gas, which is diverted into a vessel containing sulfuric acid. The ammonia reacts with the acid to make ammonium sulfate, the common nitrogen-rich fertiliser. 

The future

If all works out, the new urine-recycling process should be profitable. Analysis by Dr Pradhan’s group shows that recycling 1,000 litres of urine could yield a profit of about US$2.50. 

To make money, Dr 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 which pee goes into one part and faeces into another. Although still relatively rare, such toilets are becoming more common in Finland and Sweden.

Strange as it sounds, the excess nutrients our bodies excrete as wastes could ultimately lead to a more sustainable society.


Source:  Science News for Students, made possible with generous support from the Lemelson Foundation.