Modern wastewater treatment generates large amounts of sewage sludge. Because this sludge is rich in nutrients, it is used in Estonia as an organic fertilizer. However, its use is regulated by a decree from the minister of the environment to prevent harmful effects on the environment, as well as on human and animal health.

Margit Heinlaan, a researcher at the Environmental Toxicology Laboratory of the National Institute of Chemical Physics and Biophysics (KBFI), together with her colleagues, published the first study to examine microplastic content in water entering and leaving Estonia's wastewater treatment plants. The research focused specifically on six treatment plants of varying sizes.

"Microplastic refers to water-insoluble plastic particles ranging in size from one micrometer to five millimeters. For the first time, the amended Urban Wastewater Directive that entered into force this year includes obligations to reduce microplastic emissions. Based on the new regulations, it is important to develop local microplastic monitoring capabilities and to assess Estonia's capacity to reduce plastic pollution released into the environment through wastewater treatment processes," Heinlaan explained.

The study's results showed that 99.6-99.8 percent of plastic particles entering treatment plants are currently being captured, which Heinlaan described as very high and comparable to the efficiency levels seen in neighboring countries. The treated effluent released into the environment contained between 0.5 and 1.4 plastic particles per liter of water.

"It's important to clarify here that we examined only large particles — those over 300 micrometers. Although we found very few particles of this size in the treated wastewater, scientific data suggests that smaller microplastics, which pose a greater risk to human and environmental health, may be present in significantly higher quantities," the researcher emphasized.

In addition to particle size, what happens to the removed particles is also important. According to Heinlaan, microplastics from treated water generally end up in sewage sludge: "Unlike in most European countries, sewage sludge is not incinerated in Estonia. It is handed over to waste handlers and used in landscaping and agriculture. Based on our findings, this means that microplastics larger than 300 micrometers, which are removed from water, ultimately make their way into nature through sludge."

KBFI is currently conducting further analysis in which Heinlaan and her colleagues aim to determine the levels of microplastics and plastic additives — such as plasticizers like phthalates and their alternatives — in sewage sludge. "In the second half of this year, we will expand our research through the INTERREG Balt-Plast-Free project. In cooperation with LUT University (Lappeenranta-Lahti University of Technology), we'll be testing a pilot solution at one Estonian treatment plant to reduce microplastic content not only in wastewater, but also in sewage sludge," Heinlaan said.

Coeffect of additional substances largely unknown

According to Heinlaan, wastewater treatment plants act as intermediary stations in the movement of plastic pollution into the natural environment. "Once in the environment, plastic breaks down into increasingly smaller particles due to various factors. Plastic, being a material with a diverse chemical composition, is essentially a cocktail of polymer components and numerous additives. The solid particles are just one component of plastic pollution," the researcher noted.

Heinlaan pointed out that more than 16,000 plastic additives are known. These additives, which are released as plastic fragments and can enter organisms alongside the particles, may actually pose a greater health risk than the plastic itself, she argued. "Especially since we're dealing with a complex interaction here — one that remains largely unknown, including whether size, shape or chemical composition has the greatest impact," she added.

"Very broadly speaking, when we talk about the biological effects of plastic particles, size is one of the most critical factors. It determines where the particle ends up in the body and how reactive it is," Heinlaan said.

She noted that plastic particles are certainly making their way into gardens through sewage sludge. At the same time, she emphasized that microplastic is not found only in sewage sludge — it's also present in indoor dust, on city streets and in packaging. According to Heinlaan, completely avoiding the ingestion or inhalation of microplastics is impossible.

"Although the amended Urban Wastewater Directive specifically recommends paying attention to microplastic contamination in sewage sludge used in agriculture, current knowledge suggests that the plastic particles present in sludge do not compromise the safety of food products grown with it," Heinlaan said.

"Nevertheless, we must make every effort to limit the accumulation of microplastics in soil — a problem exacerbated, for example, by the widespread use of synthetic fabric mulch, including geotextiles," she added.

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