Plastic particles smaller than 5mm (known as microplastics) are well-documented pollutants in ocean and freshwater habitats.
The discovery of microplastics in the most remote rivers of the Himalayas and the deepest trenches of the Pacific Ocean has sparked widespread concern. But how much microplastic lies closer to home — buried in the soil where food is grown?
Our latest study estimated that between 31,000 and 42,000 tonnes of microplastics (or 86 trillion — 710 trillion microplastic particles) are spread on European farmland soils each year, mirroring the concentration of microplastics in ocean surface waters.
The cause is microplastic-laden fertilizers derived from sewage sludge diverted from wastewater treatment plants.
These are commonly spread on farmland as a renewable source of fertilizer throughout European countries, in part due to EU directives that aim to promote a circular waste economy.
As well as creating a massive reservoir of environmental microplastics, this practice is effectively undoing the benefit of removing these particles from wastewater.
Spreading microplastics onto farmland will eventually return them to natural watercourses, as rain washes water on the surface of soil into rivers, or it eventually infiltrates groundwater.
Wastewater treatment plants remove solid contaminants (such as plastics and other large particles) from raw sewage and drain water using a series of settling tanks. This produces an effluent of clean water that can be released into the environment.
The floating material and settled particles from these tanks are combined to form the sludge used as fertilizer.
We found that up to 650 million microplastic particles between 1mm and 5mm in size entered a wastewater treatment plant in south Wales, U.K., every day.
All of these particles were separated from the incoming sewage and diverted into the sludge rather than being released with the clean effluent. This demonstrates how effective default wastewater treatment can be for removing microplastics.
At this facility, each gram of sewage sludge contained up to 24 microplastic particles, which was roughly 1% of its weight. In Europe, an estimated 8 million to 10 million tonnes of sewage sludge is generated each year, with around 40% sent to farmland.
The spreading of sewage sludge on agricultural soil is widely practiced across Europe, owing to the nitrogen and phosphorus it offers crops.
U.K. farms also use sewage sludge as fertilizer. In our study, the U.K. had the highest amount of microplastic pollution within its soils across all European nations (followed by Spain, Portugal and Germany).
Between 500 and 1,000 microplastic particles are applied to each square meter of agricultural land in the U.K. every year.