Awareness of the environmental and health risks posed by microplastics has increased significantly in recent years. Microplastics enter the water cycle via household wastewater and stormwater runoff from sealed surfaces. Wastewater treatment plants play a crucial role here: as demonstrated by a team led by Günter Gruber from the Institute of Urban Water Management and Landscape Water Engineering at TU Graz and Helmut Rechberger from the Institute for Water Quality and Resource Management at TU Wien, in collaboration with the Environment Agency Austria, these facilities retain more than 95 percent of detectable microplastics. Together, the researchers have developed a standardized and practical method for determining microplastics in the influent and effluent of wastewater treatment plants.

Sampling as a Potential Source of Error
“There are now many studies on microplastics with a wide range of findings and results,” says Helmut Rechberger of TU Wien. “However, because there is still no standardized method for comprehensive analysis, the results are difficult to compare. With the analytical method we developed, comparable results can now be achieved.”

During the investigations—carried out mainly at and with the support of the Graz municipal wastewater treatment plant—the TU Graz team focused on the challenging task of collecting representative samples and obtaining solid fractions. This step is considered one of the biggest potential sources of error, as microplastic particles vary greatly in size, density, and behavior in water. As a result, they are distributed very unevenly in wastewater—some particles float on the surface, others move along the bottom, and the rest are somewhere in between. To obtain representative measurements, the researchers needed large-volume 24-hour composite samples: 100 liters at the influent and 1,000 liters at the effluent, covering fluctuations in wastewater quantities throughout the day.

Seeking Turbulence
To ensure good mixing, the team specifically looked for turbulent zones at the Graz wastewater treatment plant and collected samples there over 28 days, using stainless steel containers to avoid additional contamination. Preliminary experiments identified the ideal suction point, since sampling cannot take place across the entire depth of the water column. The approach was then validated with additional investigations at the Wiener Neustadt wastewater treatment plant.

The TU Wien team focused primarily on solid-liquid separation, sample preparation, and one of the two analytical methods used to determine microplastics. For this purpose, a disc filter system was developed that allows sequential filtration of large sample volumes—first with a mesh size of 20 µm, then 10 µm—to obtain solid fractions. The Environment Agency Austria contributed a second analytical method capable of determining not only mass but also the type of plastics and their concentrations.

High Proportion of Tire Wear Particles
“We were particularly intrigued to see that tire wear particles made up a very large fraction of the influent microplastics,” says Günter Gruber. “The good news is that our measurements show that wastewater treatment plants act as an excellent sink for microplastics. And with Austria’s mandated thermal treatment of sewage sludge from wastewater treatment plants serving more than 20,000 population equivalents—coming into effect in 2033—this microplastic will then be rendered practically harmless.”

The project was funded by the Federal Ministry of Agriculture, Forestry, Regions and Water Management (BMLUK) with 442,830 euros.

Contact at TU Graz:
Günter GRUBER
Ass.-Prof. Dipl.-Ing. Dr.techn.
TU Graz | Institute of Urban Water Management and Landscape Water Engineering
Tel.: +43 316 873 8373
guenter.gruber@tugraz.at

Contact at TU Wien:
Helmut RECHBERGER
Univ.-Prof. Dipl.-Ing. Dr.techn. Dr.h.c.
TU Wien | Institute for Water Quality and Resource Management
Research Unit Waste and Resource Management
Tel.: +43 1 58801 22645
helmut.rechberger@tuwien.ac.at