Studying the Concentration of Microplastics in Wastewater Treatment Plants

Microplastics released from wastewater treatment plants remain a largely invisible but significant source of coastal pollution, according to a new study published in the Journal of Environmental Management.¹ Researchers from the Institute of Environmental Assessment and Water Research (IDAEA-CSIC) report that even well-performing treatment facilities can discharge tens of billions of microplastic particles annually, with emissions strongly shaped by seasonal tourism and weather patterns.¹

Microplastics are tiny plastic particles that are less than 5 mm in size.² They pose a hazard to the environment because they contaminate estuarine ecosystems, as well as other ecosystems like forests and salt marshes.^2,3 As a result, researchers have been focused on developing new methods that could accurately detect microplastics in the environment.

In this study led by researcher M. Silvia Diaz-Cruz of IDAEA-CSIC, the research team examined municipal wastewater treatment plants (WWTPs) serving the coastal towns of Cambrils and Palamós in Catalonia, Spain. Using 24-hour composite samples collected in January, May, and September 2023, the team assessed microplastic concentrations, removal efficiencies, and links to environmental and anthropogenic drivers such as temperature, precipitation, and population influx.¹ The findings highlight how seasonal dynamics can overwhelm conventional wastewater infrastructure, particularly in tourism-dependent Mediterranean regions.¹

“These findings emphasize the need for seasonally resolved monitoring and adaptive management strategies considering transient population loads and hydrological variability,” the authors wrote in the study.¹

Wastewater treatment plants are widely recognized as critical interception points for microplastics before they reach rivers and coastal waters.^1,4 However, this study shows that their effectiveness varies over time. The influent concentrations of microplastics sized 25 µm and larger ranged from 30.0 ± 8.0 to 75.0 ± 18.0 particles per liter, whereas secondary effluents still contained between 6.7 ± 2.0 and 20 ± 5.0 particles per liter.¹ The researchers noted that the highest concentrations were consistently measured during warmer months.

The study also evaluated removal efficiencies. Using conventional treatment processes, the removal efficiencies ranged from 73.3% to 86%.¹ However, the persistence of microplastics in treated effluent, particularly during peak tourism seasons, underscores the limitations of existing systems. Estimated annual emissions reached up to 131 billion particles from the Cambrils plant and 91 billion from Palamós, which is comparable to those reported for larger urban facilities elsewhere in Europe.¹

To generate these data, the researchers combined oxidative digestion and density separation with stereomicroscopy for particle counting and Fourier transform infrared (FT-IR) imaging for polymer identification.¹ The researchers classified particles with a spectral match above 70% to certify data integrity.¹

Further investigation was done to analyze the microplastics that were uncovered. The researchers found that polypropylene (PP) and polyethylene (PE) were the two most common materials at both sites and comprised the majority of microplastics. This finding makes sense as it reflected common sources such as packaging, synthetic textiles, and consumer products.¹

Particle shapes were dominated by fragments and fibers, while color and morphology profiles suggested distinct local inputs. As a result, different microplastic signatures were observed in both regions. For example, In Cambrils, the microplastic signature was linked primarily to urban and tourism-related activities, as well as proximity to a major highway.¹ In Palamós, a stronger contribution from fishing and marine-related activities was evident, consistent with the town’s port and maritime economy.¹

The key takeaway from this study is that microplastic concentrations were heavily influenced by temperature. At the site in Palamós, precipitation also played a key role in influencing microplastic concentrations. The authors suggested that because Palamós’s maritime catchment area and partly separative sewer system allows stormwater runoff to introduce more particles during rainfall events, it is logical that rainfall would result in the area seeing higher microplastic concentrations.¹

The study addresses a gap in microplastics research, which has often focused on large metropolitan plants or single time points. By integrating environmental conditions with social factors such as tourism, the work demonstrates that medium-sized coastal WWTPs can represent a disproportionate source of microplastic emissions during certain periods of the year.¹

References

1. Contreras-Llin, A.; Martinez-Landa, L.; Valhondo, C. et al. Microplastics in Mediterranean Coastal Wastewater Treatment Plants: Seasonal Trends Driven by Tourism and Weather Conditions. J. Environ. Manag. 2026, 397, 128173. DOI: 10.1016/j.jenvman.2025.128173
2. Wetzel, W. Microplastics Found in Deepest Reaches of Central Indian Ocean. Spectroscopy. Available at: https://www.spectroscopyonline.com/view/microplastics-found-in-deepest-reaches-of-central-indian-ocean (accessed 2026-02-10).
3. Wetzel, W. Microplastic Menace in India’s Pichavaram Mangroves Revealed in New Study. Spectroscopy. Available at: https://www.spectroscopyonline.com/view/microplastic-menace-in-india-s-pichavaram-mangroves-revealed-in-new-study (accessed 2026-02-10).
4. Gkatzioura, A.; Zafirakou, A.; Zorpas, A. A. Wastewater Treatment Plants’ Contribution in Microplastics’ Contamination of Surface Water. Effluent Concentration and Detection Techniques Review. Desalin. Water Treat. 2021, 227, 26–33. DOI: 10.1016/dwt.2021.27287