Sludge produced by sewage treatment processes is high in nutrients such as phosphorous and nitrogen, making it an excellent fertilizer source for agriculture. However, not everything it contains is good for the environment, with a new study demonstrating how the material acts as a vehicle for massive amounts of tiny plastic fragments to enter soils, so much so that the authors speculate that Europe's farms may be the world's largest reservoir for microplastics pollution.
For both large-scale agriculture operations and home gardeners, sewage sludge is an appealing and sustainable source of fertilizer. However, studies are beginning to show that its contents may not be entirely safe for the environment or living organisms.
A study published last year that examined home fertilizer products discovered dangerous levels of toxic PFAS "forever chemicals" in all samples. According to the findings, standard sewage treatment methods do not break down these persistent chemicals, and because sludge is widely applied to lands across the United States, it introduces massive amounts of them into food crops and waterways.
This new study, conducted by scientists from Cardiff University and the University of Manchester, focused on European farmlands and the risks posed by sewage sludge fertilizers. The work entailed analyzing samples from a wastewater treatment plant in Newport, South Wales, which treats sewage from a population of approximately 300,000.
This demonstrated that the plant was collecting larger plastic particles ranging in size from 1 to 5 mm with a 100% strike rate, preventing them from slipping into the waterways. However, each gram of the sewage sludge produced by this process was found to contain up to 24 microplastic particles, amounting to about 1% of its total weight.
The scientists then extrapolated this using data from the European Commission and Eurostat on the use of sewage sludge as fertilizer across the continent. This meant that between 31,000 and 42,000 tonnes of microplastics, or trillions of particles, were applied to European soils each year. According to the authors, this is comparable to the concentration of microplastics in ocean surface waters.
"Our research questions whether microplastics are actually being removed at wastewater treatment plants at all, or whether they are effectively being shifted around the environment," said James Lofty, lead author of the study from Cardiff University's School of Engineering. "Water companies' clear lack of strategy to manage microplastics in sewage sludge means that these contaminants are transported back into the soil and will eventually return to the aquatic environment."
The findings provide new insights into how microplastics migrate through the environment, but they may not be all that surprising given recent research in the field.
A 2018 study discovered microplastics in human stool samples from all over the world, and scientists have also discovered particles of plastic in the human bloodstream recently and deep within the lungs for the first time.
Other studies have shown that microplastics in wastewater treatment plants can promote the growth of superbugs and transport dangerous pathogens far out to the sea.
"Our findings highlight the magnitude of the problem across European soils and suggest that the practice of spreading sludge on agricultural land may make them one of the largest global reservoirs of microplastic pollution," Lofty said. "At the moment, there is no European legislation that limits or controls microplastic input into recycled sewage sludge-based on microplastic loads and toxicity."
