**1. Introduction**

WWTPs are a major indirect source of MP emissions into the environment, due to the daily discharge of large quantities of MPs, from agricultural, industrial or urban activities, to the sewage system [1–3]. At the household level, this pollution mainly comes from the use of products that containing MPs, namely cosmetic and personal care products, and also fibres generated during laundry [4–6]. In addition, MPs can be originated from the weathering and fragmentation of plastics due to disposal mismanagement or by the wear and tear of plastic items [7–9]. These microplastics can enter to the sewage system by surface runoff or stormwater, either because they are on the ground surface or deposited from the atmosphere [10–12]; therefore, wastewater could contain a high number of MPs, specifically, the MP concentration reported in WWTPs ranged between 0.28 and 3.14·10<sup>4</sup> particles/L [13]. Although WWTPs can frequently achieve removal efficiencies of MPs up to 90%, this is insufficient because large quantities of microplastics are still being released into rivers and oceans [13–15].

It has been reported that most MPs removed during the wastewater treatment are accumulated in sludge [16]. So far, the reported ranges of MP concentration in wet and

**Citation:** Menéndez-Manjón, A.; Martínez-Díez, R.; Sol, D.; Laca, A.; Laca, A.; Rancaño, A.; Díaz, M. Long-Term Occurrence and Fate of Microplastics in WWTPs: A Case Study in Southwest Europe. *Appl. Sci.* **2022**, *12*, 2133. https://doi.org/ 10.3390/app12042133

Academic Editor: Bart Van der Bruggen

Received: 27 December 2021 Accepted: 12 February 2022 Published: 18 February 2022

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dry mixed sludge were 400–7000 and 1500–170,000 particles/kg, respectively [17–20]. Furthermore, the repeated application of sludge in agriculture as soil amendment is a potential problem, as it favours the excessive and unavoidable accumulation of MPs in the farmlands. It is estimated that the use of sludge as fertilizer releases in European agricultural lands between 63,000 and 430,000 tons of MPs per year [21,22]. MPs not removed from the wastewater during the treatment processes are finally released into the aquatic environment; in particular, the abundance of MPs in the effluent of urban WWTPs ranges between 0.01 and 297 particles/L [13]. MPs emitted to the environment become a potential risk, not only to the ecosystems, but also to human health, since they can be bioaccumulated through the trophic chain [23–26].

Several chemical, physical and biological processes take place in WWTPs to achieve high-quality effluent water. Each treatment plant uses its own technologies depending on different factors (the subsequent reuse of water, the characteristics of wastewater, the place where the effluent is discharged, etc.) [3]. When the wastewater treatment includes dynamic membranes (DMs) or membrane bioreactor (MBR), MP removals of 99% or even higher have been reported [18,27–30]. The major drawback is the high cost of implementing and maintaining these technologies. Surprisingly, there are some works that reported similar removal efficiencies employing lower cost technologies, such as conventional activated sludge (CAS) and sequencing batch reactor (SBR) [18,31]. In fact, removal efficiencies in the range between 96–98% have been reported from WWTPs that used that kind of technologies. It is necessary to point out that most works have calculated the removal efficiencies just by analysing a few samples, which can contribute to the dispersion of efficiencies. Analysing the WWTPs performance for extended periods would be necessary to stablish accurate conclusions. Therefore, in this work, the performance of wastewater treatment processes was evaluated in a WWTP sited in Southwest Europe over a 12-month period. The aim of the study is increasing the knowledge on the behaviour, fate and elimination of microplastics in the different stages of treatment throughout the year. Furthermore, as far as we know, it is the first study to analyse the effect of a double consecutive decantation (secondary treatment), as well as the use of a lamellar settler in the tertiary treatment.

#### **2. Materials and Methods**
