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The city of Tulear is undergoing environmental degradation. The wastewater is heavily polluted, the average values observed for biological oxygen demand (282,347 ± 76,907 mg.L-1), chemical oxygen demand (579,302 ± 161,102 mg.L-1), and suspended solids (859,114 ± 816,707 mg.L-1) are above the standard discharge values. This study allowed us to design a treatment system and to make a technical, economic, environmental and social assessment of the treatment plant. Thus, the treatment method chosen is the physico-chemical process of coagulation – flocculation –filtration. Indeed, for a population of 368,020 inhabitants, the average hourly discharge rate is estimated at 662.436 m3/d. In addition, the number of bars for grids n°1 and n°2 are respectively 34 and 140, the surface of the sedimentation basin is estimated at 204,5m2, the optimal dose of coagulant (Al2(SO4)3) and flocculant (polyacrylamide) for the coagulation-flocculation basin are respectively 300 mg.L-1and 2 mg.L-1and this gives a considerable drop in suspended solids of the order of 15,22 mg.L-1after treatment. For the settling basin, the volume of clarifier is estimated at 306,68m3, finally the sludge produced is estimated at 1613,66 m3.

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