The Moaña wastewater treatment plant (WWTP) manages to prevent more than 90% of the microplastics present in wastewater from reaching the Ría de Vigo. This is the main conclusion of the Clear-Edar project, led by Víctor Hernando, researcher at the University of Vigo’s Marine Research Centre, who also estimates that around two tonnes of these pollutants end up reaching the marine environment each year through treated effluent — evidence that treatment plants are a fundamental barrier, but insufficient on their own to eliminate this type of emerging pollution.
The study thus confirms the role of the Moaña Wastewater Treatment Plant (WWTP) as essential infrastructure for protecting the Ría de Vigo, an area of high ecological, fishing, and socioeconomic value. With an overall retention efficiency of over 90%, the plant’s performance is comparable to figures reported in international scientific literature.
The Vigo Oceanographic Centre of the Spanish Institute of Oceanography (IEO-CSIC) collaborated on the project, handling the spectroscopic characterization of samples, along with the University of Vigo’s ETEA and CIES research groups.
An X-ray of plastics’ journey through the treatment plant
To understand the fate of plastics throughout the treatment process, the scientific team analyzed four points in the facility: the influent inlet, the screening stage, the sludge generated during biological treatment, and the final effluent.
Analysis of the screening stage — the solid fraction retained at the plant’s entrance — revealed a dominant presence of sanitary textiles, mainly wet wipes, accounting for around 70% of the total dry mass. This percentage remained practically constant across both summer and winter sampling campaigns, identifying this type of waste as the main structural problem in urban sanitation.
The study also found marked seasonal variability. In summer, a higher number of light fragments were detected, likely linked to increased tourist pressure, while in winter larger debris appeared, related to runoff caused by episodes of intense rainfall.
The waste reaching the ría remains significant
Data gathered by the project indicate that up to 47 tonnes of microplastics could reach the Moaña WWTP each year. Most of that load is retained during the treatment process, particularly in the sludge — where approximately 11.5 tonnes accumulate annually — and also in the screening stage. Only a small fraction manages to pass through the system and is discharged into the ría, although its environmental relevance remains significant.
Average concentrations of around 1,180 particles per litre were recorded in the inlet water, dominated by fragments and fibres of everyday polymers such as polyethylene (PE), low-density polyethylene (LDPE), and polyethylene terephthalate (PET). Once treatment is complete, that concentration drops to around 91 particles per litre, confirming the facility’s high retention capacity.
New protocols and opportunities for the circular economy
Besides assessing the plant’s efficiency, the project contributed to refining protocols for digesting and separating microplastics in complex matrices such as sludge and wastewater. These advances provide a solid methodological basis for future research, in a context where the new EU Urban Wastewater Treatment Directive (UWWTD 2024/3019) establishes the need to move toward harmonized systems for controlling these pollutants.
In terms of waste recovery, vermicomposting trials showed that the organic fraction of the screenings can biodegrade, leaving only non-degradable materials behind. This result opens the door to incorporating new circular economy strategies into the management of treatment plants.
From research to social awareness
The project also included an important outreach and educational component, with activities aimed at citizens to explain the journey of microplastics “from the toilet to the sea” and to promote behavioural change in the use and disposal of hygiene products and everyday plastics.
The conclusions of Clear-Edar highlight that the fight against plastic pollution requires a combination of effective infrastructure, technological improvements, prevention at the source, and greater social awareness. The study is one of the first comprehensive assessments of microplastics at a small-scale treatment plant in Galicia and provides strategic information to guide decision-making on sanitation and the protection of the rías, with the aim of extending this line of research to other facilities in the province.
CIM holds the Cigus accreditation awarded by the Regional Government of Galicia, which certifies the quality and impact of its research, and its activities are co-funded by the European Union through the ERDF 2021–2027 programme.
