It’s all due to the high levels of these particles in sewage-sludge-derived fertilizers.

Researchers at Cardiff University estimate that anywhere between 31,000 and 42,000 tons of microplastics (or 86 – 710 trillion microplastic particles) find their way into Europe’s farmlands every year. These quantities mean that the average plot of farmland in the old world mirrors the microplastic levels of ocean surface waters, they add.

As for the source of these plastics, the team points to sewage sludge, a material that is commonly used as feedstock for fertilizers on farmlands across Europe. They estimate that around 1% of the weight of sewage sludge is made up of microplastics.

Heavily contaminated

The UK has potentially the highest amounts of microplastic contamination in its soil, with between 500 and 1000 particles per square meter of agricultural land applied per year, judging from mean fertilizer (from sewage sludge) use annually. Spain, Portugal, and Germany follow.

Sewage sludge is used as fertilizer for agricultural lands as it is a renewable source of nitrogen, phosphorus, and other nutrients that help crops grow. In part, fertilizers derived from sewage sludge are popular in Europe due to EU directives that promote the diverting of such sludge away from landfill or incineration towards energy production or agricultural uses.

The team explains, however, that microplastics that find their way into farmlands from such fertilizers will end up back into watercourses through water run-off or infiltration into groundwater. With their diminutive size (microplastic particles are less than 5mm in diameter), such particles pose a significant threat to wild plant and animal life; they are ingested through contaminated water and food and can carry toxic chemicals or dangerous pathogens into tissues such as blood or organs. Here they build up and transfer from prey to predators, with the potential to impact animals across the food chain.

“Our research questions whether microplastics are in fact being removed at wastewater treatment plants at all, or are effectively being shifted around the environment,” said lead author of the study James Lofty, from Cardiff University’s School of Engineering. “A clear lack of strategy from water companies to manage microplastics in sewage sludge means these contaminants are transported back into the soil and will eventually return to the aquatic environment.”

The study is based on samples taken from the Nash Wastewater Treatment Plant in Newport, South Wales. The plant serves a total population of around 300,000 locals. Based on these samples, the team reports that sewage sludge incoming to the site contained up to 24 microplastic particles per gram, or around 1% of its weight. The plant was 100% effective at removing large microplastic particles, those between 1-5 mm in diameter, from the sewage it processes and releases.

Measurements performed on these samples were then used to assess the impact of using sewage sludge as fertilizer across Europe, with usage figures for this type of fertilizer from the European Commission and Eurostat. The team further notes that, as their analysis of microplastics did not include particles under 1mm in size, it’s likely that the overall levels of microplastics finding their way into Europe’s fields are a lot higher than their estimates.

“Our results highlight the magnitude of the problem across European soils and suggest that the practice of spreading sludge on agricultural land could potentially make them one of the largest global reservoirs of microplastic pollution,” James Lofty continued. “At present, there is currently no European legislation that limits or controls microplastic input into recycled sewage sludge based on the loads and toxicity of microplastic exposure.”

“Efforts should be made to increase standardised monitoring of microplastic concentrations in sewage sludge and agricultural soils, which would provide a more accurate picture of contamination levels in soils across Europe.”

The paper “Microplastics removal from a primary settler tank in a wastewater treatment plant and estimations of contamination onto European agricultural land via sewage sludge recycling” has been published in the journal Environmental Pollution.