Liquid gold: could we turn our pee into fertiliser?

After our bodies absorb nutrients from our food, they excrete the rest – mainly via urine. An adult produces about 500 litres a year. This liquid gold is very rich in nitrogen, phosphorus and potassium, three compounds vital for plant growth.

Ancient civilisations from Greece to China to Rome used it as fertiliser for crops, alongside faeces.

Synthetic fertilisers have largely replaced natural fertilisers such as urine. Half the world’s population now relies on food grown using synthetic fertilisers. Urine is still used as fertiliser by smallholder farmers who cannot afford synthetic fertilisers or want a sustainable alternative.

This year, the Iran war has made synthetic fertilisers more expensive. If the era of cheap fertilisers is ending, urine-based fertilisers could make a comeback. Call it “peecycling”.

Of chamber pots and nightsoil

Before modern toilets, humans were a lot more intimately acquainted with the waste they produced. Peasants and smallholders would store their collective waste in cesspits. In cities, people used privies or chamberpots.

As cities grew, so did the volume of waste. While people slept, nightsoil workers would go door-to-door collecting human waste. Often, it would be transported for use on farms.

In the 19th century, modern central sewage greatly improved sanitation and cut rates of infectious diseases. But it also changed how we thought about pee and poo from resource – to waste.

From guano to synthetic fertilisers

During the 19th century, nations competed to gain access to a new source of fertiliser – guano, the compressed, nitrogen-rich droppings of seabirds. But the resource was limited. Guano islands were progressively mined out.

In the early 20th century, chemists invented synthetic fertilisers as an alternative.

These fertilisers boosted crop yields. But they also created new social and environmental problems.

Synthetic nitrogen fertilisers are produced using the industrial Haber-Bosch process. It uses lots of energy and relies on natural gas, a potent fossil fuel.

Synthetic phosphorus fertilisers are made by treating phosphate rock, a finite resource mined in only a few places. One of the largest reserves is in Western Sahara, a disputed territory long occupied by Morocco.

If synthetic fertilisers are applied at scale without good nutrient management, they can trigger toxic algal blooms and mass fish kills.

Fertiliser shocks

Australian soils are heavily weathered and nutrient-poor in most places.

To produce as much food as it does, Australia has to import 3.5-4 million tonnes of synthetic fertilisers every year. Almost no fertilisers are produced locally.

Relying on global markets works when times are good. But at a time of escalating global shocks, it is a huge vulnerability.

One answer could be to look for sustainable local sources of nutrients for the plants we depend on – such as urine.

Flushing liquid gold?

Urine represents less than 1% of all wastewater, but contains almost all its nitrogen and half its phosphorus.

Once mixed with wastewater, these nutrients are much harder to access.

To get enough urine to make fertiliser would involve using separate plumbing in homes, public toilets and at events. Urinals are an existing source.

When mixed with wastewater, the nitrogen in urine can turn into nitrous oxide, a potent greenhouse gas, or ammonia. Wastewater plants release the greenhouse gas emissions equivalent of the global aviation industry.

If urine is diverted from wastewater, authorities could save on wastewater treatment – and cut emissions.

How do we capture urine?

At a small scale, raw urine can be captured using specialised urine-diverting toilets or urinals. Urine can be applied directly to your home garden, as long as sensible guidelines are followed.

To scale up urine-based fertiliser, we will have to tackle bigger issues such as the chance of pathogens and pharmaceuticals being found in urine, as well as inconsistent plumbing codes.

These can be managed with the right treatments and governance.

Urine nations?

Urine fertilisers have gained popularity in recent years.

In the United States, the Rich Earth Institute has lead community peecycling programs for over a decade. In Europe, Swiss startup VunaNexus uses diverted urine to make a sustainable fertiliser.

Sporting arenas in the United Kingdom, Sweden and South Africa are trialling the approach. And in Australia, researchers are trialling urine fertilisers in Brisbane and Sydney parklands.

One of the biggest barriers to scaling up urine-based fertilisers is the ick factor. Some people find the idea gross.

Fortunately, this can change. Researchers have been testing ways to challenge the stigma around urine. Our new podcast examines how rethinking the humble toilet can turn waste into fertiliser.

For centuries, people saw urine as the vital resource it is. The fact we can help grow crops using something from our bodies is remarkable. In an increasingly uncertain world, it might well be necessary.