**2. Materials and methods**

#### **2.1 Effluent origin**

The treated wastewater is collected at the outlet of the Wastewater treatment plant (WWTP) of Mahrès (40 km south of the city of Sfax) which mainly treats domestic wastewater. Samples are collected during the period from January to May. After each companion, part of the sample is stored at −4° C for characterization. The WWTP of Mahrès treats the urban wastewater of the city and that of the Chafar seaside area. It is designed for a capacity of 13,000 equivalent inhabitants, which corresponds to a daily flow of 780 m3 /day. The average daily flow was 800 m3 /day with peak flow rates exceeding 1400 m3 /day, especially in summer. The station treatment process includes pretreatment (grit screening), biological treatment with activated sludge in an oxidation channel. A settling basin and sludge treatment (thickening, de-watering). The treated wastewater is, at the end, rejected in the sea.
