## 2 Materials and Methods  
### A Description of the Working Environment: The DWTP of Turin  
Societa Metropolitan Acque Torino (SMAT) is the company in charge of water cycle management in the Metropolitan Area of Turin. SMAT operates drinking, and wastewater, treatment plants, as well as the distribution networks in the area, supervising 291 municipalities and supplying a population of about 2.3 million inhabitants. The description of the treatment steps of the studied DWTPs reported in this work (shown in Figure 1) is below. The studied DWTP treats a mix of water from the river Po (site C) and a lagoon located about 10 km upstream (sites A and B, for the lagoon's inlet and outlet, respectively). The lagooning pre-treatment was included in the studied DWTP to improve the characteristics of treated water and reduce the amounts of chemical reagents needed along with the different processes. Water from sites B and C is mixed and passed through a wire mesh filter (6 mm\({}^{2}\) mesh size), which removes most of the coarse contaminants and sediments (site D) before flowing into the treatment plant. The river bed is pumped (ca. 3.6 m\({}^{3}\) s\({}^{-1}\) flow) through 100 cm diameter steel pipelines into a static horizontal flow pre-settling basin (designed to handle a flow of 2 m\({}^{3}\) s\({}^{-1}\)), which consists of a circular pool of 33 m  
Figure 1: Sampling sites from SMAT DWTP, (**A**) scheme, (**B**) air pictures (taken from Map data ©2019 Google). Sampling sites are denoted with red colour letters. in diameter and 5.15 m high (4300 m\({}^{3}\) capacity and effective surface area of 850 m\({}^{2}\)) and equipped with a rotary dredge for the mechanical removal of sludge, where additives (iron or aluminium chloride salts) are used to promote the sedimentation process. Powdered activated carbon (PAC) is also added to remove emerging contaminants in this step. The sludge, accumulated at the bottom, is evacuated by two electric pumps with a capacity of 35 L s\({}^{-1}\). At the outlet of the pre-settling basin (site E), the treatment plant is divided into three different lines, Po1 and Po2 (identical treatment lines hereafter named as Po1/Po2, with a total output of 1.1 m\({}^{3}\) s\({}^{-1}\)) and Po3 (with an output of 1.5 m\({}^{3}\) s\({}^{-1}\)). For the two identical treatment lines, Po1/Po2, a pre-chlorination step takes place using chlorine dioxide (ClO\({}_{2}\), produced on-site by the combination of HCl and NaClO\({}_{2}\) with an output of 12 kg h\({}^{-1}\) and conveyed to the various stabilisation system inlet points via PVC conduits) and sodium hypochlorite (NaClO, stored in fibreglass reinforced resin reservoirs and added to the treatment cycle by piston pumps via PVC pipes). Aluminium polychloride (pAlCl\({}_{3}\)) flocculant is also administrated here with a capacity of 75-750 L\({}^{-1}\) h\({}^{-1}\). Subsequently, the water is headed to the clarification step (site L), consisting of an \"Accelerator\" type sludge recirculation tank (diameter ca. 25 m, 1600 m\({}^{3}\) volume, and maximal output of 500 L s\({}^{-1}\)). Finally, the treated water is filtered on 12 parallel lines units of granular activated carbon (GAC, filtering layer of 800 cm) and afterwards introduced into a tank where the final disinfection step with ClO\({}_{2}\) occurs (site M). Regarding the other parallel line, Po3, the water undergoes an ozonation process (site F). O\({}_{3}\) is produced by four generators made up of 558 dielectric tubes with a production capacity of 8 kg h\({}^{-1}\) each; the O\({}_{3}\) is blown into the water via porous ceramic plates arranged along the bottom of the contact chambers (a concentration of 1-2 mg L\({}^{-1}\) of O\({}_{3}\) and a contact time of 5-10 min is finally reached). Right after ozonation, the water is transferred to three parallel CYCLOFOC clarifiers (where the precipitation of the sludge formed takes place promoted by the addition of pAlCl\({}_{3}\) and microsand) with an overall capacity of 1.5 m\({}^{3}\) s\({}^{-1}\).