*2.2. Sampling Points*

To obtain representative and homogeneous samples, the water was collected in turbulent areas to prevent the heavier MPs to sediment. Sampling points are indicated by an asterisk in Figure 1, i.e., after the screening systems (influent), effluent from secondary treatment, effluent from tertiary treatment and dehydrated sludge. To collect the water, it was pumped through a sieve module assembled in a specific sampling device (Figure S1). This device is made up of four mesh stainless steel filters (CISA Sieving Technologies) of 150 mm of diameter and the following slot sizes: 500, 250, 100 and 20 μm (placed from the largest to the smallest one). Thus, MPs contained in the sampled water were classified by size and retained in the corresponding sieve. The flow rate chosen for sampling was 10 L/min which was maintained during approximately 30 min (or until the solids clog the sieves) at each collection point. MPs collected were dragged with distilled water and stored under refrigeration until further processing. The volumes of wastewater sampled at each sampling point each month are detailed in Table S1.

Dewatered sludge samples were also stored under refrigeration. In order to express MP concentration on dry weight basis (*w*/*w*), a gravimetric method was used to determine the moisture content of each sample of sludge by triplicate.

## *2.3. Pre-Treatment of Samples*

Water samples were stored in an oven at 90 ◦C to dryness. After that, the organic matter was degraded by treating the samples with Fenton's reagent (20 mL of solution of Fe(II) at pH 3 with 20 mL of H2O2 50%) at room temperature, during 30 min. Once digested, samples were left at room temperature for 24 h to allow the residual hydrogen peroxide to evaporate and, then, they were stored in an oven at 90 ◦C to dryness (10 h). MPs were isolated from the remaining inorganic impurities by density using a solution of ZnCl2 (d = 1.6 g/mL) (97% purity, VWR), so that supernatant was filtered under vacuum using a glass microfiber filter (Whatman, diameter 47 mm, pore size of 0.7 μm).

Sludge samples (5 g) were oxidised during 24 h with 30 mL of hydrogen peroxide (H2O2, 50%). This process was carried out twice. The rest of the procedure was the same as that employed for water samples.

Distilled water and zinc chloride solution employed in the pre-treatment samples were previously filtered using a glass microfiber filter (Whatman, diameter 47 mm, pore size of 0.7 μm) to avoid MP contamination.

## *2.4. Microplastic Analysis*

Filters with MPs were examined under a semiautomatic stereomicroscope (Leica M205FA) with a high-resolution colour digital camera attached (Leica DFC310FX) to process images with a maximum resolution of 1392 × 1040 pixels (1.4 Mpixels CCD). It is used for the quantification of MPs and the analysis of colour and shape of microparticles [28,32–34].

To determine the chemical composition of microplastics, an FTIR spectrophotometer coupled to a microscope with an imaging system (Varian 620-IR and Varian 670-IR) with three detection systems is used [35]. Samples were analysed in the mid-infrared of 4000–400 cm<sup>−</sup>1, a range in which the most typical bands of plastic materials are identified. The identification of functional groups and molecular composition of polymeric surfaces was carried out using the list of absorption bands of sixteen polymers described by Jung et al. (2018) [36].
