Résumé
The Aghien lagoon, one of the largest freshwater reserves near Abidjan, is one of the most interesting alternatives for producing drinking water to make up for the shortage of water in the District of Abidjan. However, in recent years, the Aghien lagoon catchment area has undergone significant anthropisation, which is likely to compromise the long-term use of this hydrosystem for drinking water production. The aim of this work is to characterise the quality of the water in the Aghien lagoon and its tributaries (Bété, Mé and Djibi), and also to optimise the removal of dissolved organic matter from the water by coagulation-flocculation. Water quality monitoring is based on a seasonal sampling plan for lagoon and river water. In addition to conventional analyses (conductivity, pH, dissolved organic carbon, ammoniacal nitrogen, nitrates, etc.), optical methods (UVvisible spectrometry and fluorescence) have been applied to characterise soluble organic matter in order to better understand its origin. Finally, water treatment tests using coagulation-flocculation were carried out. The results of the physico-chemical analyses provide a clear diagnosis of the current state of pollution in the Aghien lagoon, and the contribution of the main tributaries in terms of pollutant flows. In rural areas, watercourses are impacted by pollutants of agricultural origin. Lagoon waters are of fair quality. The Djibi river, one of the main tributaries of the lagoon, is of poor quality in terms of its low dissolved oxygen content (11%) and high turbidity (99 NTU), total nitrogen (7 mgN.L-1) and total phosphorus (0.5 mg P.L-1) values linked to the influence of its more anthropised catchment area in the northern part. The results of the organic matter characterisation indicate a spatio-temporal variation in dissolved organic carbon and chemical oxygen demand, with significant values in the Mé river. SUVA values ≥ 3 L.m-1 .mg-1 in the Mé, the Aghien lagoon and the Bété indicate that the organic matter is of allochthonous origin, consisting of humic substances. 3D fluorescence spectrometry shows that soluble organic matter is composed of fluorophores (A, C and T). The main component of the organic matrix in the waters of the Mé, the Aghien lagoon and the Bété is representative of humic substances (A and C). The waters of the Djibi are characterised by the predominance of tryptophan-type fluorescent compounds. Humic substance-type compounds (Peaks A) are also observed, but at a lower intensity. The results of the laboratory coagulation-flocculation tests carried out on the waters of the Aghien lagoon and those of the Mé river show the particular importance of the coagulation-flocculation pH and the concentration of alumina sulphate in optimising the removal of organic matter. At pH 5 - 5.5, good elimination of organic matter was observed at rates of 77% and 71% respectively for the waters of the Aghien lagoon and those of the Mé river. These reductions were obtained for coagulant doses of 60 and 50 mg.L-1 respectively.