STPs.

Wastewater at all the sewage treatment plants (STPs) is subjected to secondary biological treatment with activated sludge (Table 1). However, the following three types of activated sludge treatment were involved in the study: conventional activated sludge (medium charge), used by the Sousse-South and Dekhila STPs; extended aeration oxidation ditch activated sludge (low charge), used by the Sahline and Kairouan STPs; and extended aeration activated sludge (low charge), used by Jammel STP. All the STPs follow the legal requirements of the Tunisian law included in decree no. 2005-1991 (11-07-2005) (24) on environmental impact assessment and the regulations in NT 106.02 (20-07-1989) (25) on the quality of discharges according to the nature and particularities of the receiving environment (physicochemical, bacteriological, and micropollutants) and in NT 106.03 (06-1989) (26) on the reuse of treated wastewater.