*2.2. Sample Collection*

Samples were taken in three di fferent wastewater treatment plants from Gran Canaria island (Spain). Two use conventional treatment technologies (C-WWTP1 and C-WWTP2) and receive wastewater from highly-populated areas of the island, located in the northeast and the southeast, respectively. C-WWTP1 has a secondary treatment of activated sludge and a tertiary system based on microfiltration and reverse osmosis. This WWTP treats the water of a population equivalent to 134,000 inhabitants. In C-WWTP2 only the tertiary process based on ultra-filtration was evaluated in order to know the possible risk of the produced water. This WWTP treats the water of a population equivalent to 200,000 inhabitants. The third WWTP surveyed is based in natural treatments (N-WWTP) and is located in a rural zone of the island. It is based in two constructed wetlands (CWs), the first one is a vertical flow wetland and the second one is a planted sub-superficial horizontal flow wetland. This WWTP was designed to treat wastewater equivalent to 500 inhabitants but nowadays it treats a higher volume of wastewater with grea<sup>t</sup> results.

In C-WWTP1, the samples were taken in the influent of the plant (point A1), after the biological treatment (point A2), after the microfiltration process (point A3) and lastly, in the final e ffluent of the plant, after the reverse osmosis process (point A4). In C-WWTP2, the samples were taken before and after the electrodialysis reversal process (points B1 and B2). Finally, in N-WWTP, the samples were taken in the influent (point C1) and after each process of the treatment: Imho ff tank (point C2), vertical flow wetland (point C3), and horizontal sub-superficial flow wetland (point C4) (Figure 1).

The samples were taken monthly for two years from July 2017 to June 2019, at the same time slot and collected in rinsed amber bottles of 1 L. Samples were filtered using 0.65 μm polyvinylidene fluoride (PVDF) membrane filters from Merck Millipore (Cork, Ireland), and acidified to pH below 3 using hydrochloric acid and stored in the dark at 4 ◦C to inhibit microbial activity.
