This study aimed to determine the optimal conditions for organic matter removal from wastewater by Eichhornia crassipes (E.C). As a matter of fact, a complete factorial design was used to determine the effect of residence time (X), plant density (X) and initial chemical oxygen demand (COD) concentration (X) on the phytoremediation process. The process's performance was measured on COD (Y), NH (Y) and PO (Y), with the results indicating a reduction of 8.59-81.71% of COD (Y); 22.53-95.81% of NH (Y) and 0.54-99.35% of PO (Y). Then, the first-order models obtained for COD, NH and PO removal were validated using different statistical approaches such as statistical and experimental validation. Moreover, multi-response optimization was carried out through different scenarios. On the whole, the results obtained indicated that two serial ponds are required for an optimum organic matter removal by Eichhornia crassipes. Indeed, for the first pond, a residence time of 15 days is needed with a plant density of 60 ft/m and an initial concentration of about 944 mg/L. The second was the same residence time as the first with similar plant density of 60 ft/m and an initial load 192 mg/L (> 200 mg/L). Optimal organic matter removal from wastewater using Eichhornia crassipes requires two ponds arranged in chain.

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La prise de conscience collective du pouvoir létal du mercure suite aux intoxications massives au Japon et en Irak ont conduit à l’adoption de la convention de Minamata sur le mercure que le Bénin a signé et ratifié. Le présent article fait la synthèse des connaissances sur son émission, sa toxicité en milieu aquatique avec un accent sur la contamination aquatique au Bénin. Pour y parvenir, des publications scientifiques et les archives des ministères de l’environnement et de la santé au Bénin ont été consultées. Il ressort que, la production artisanale à petite échelle de l’or et la combustion de charbon constituent les principales sources d’émission de mercure dans le monde avec respectivement 37,1% et 24,2% des émissions anthropiques. Au Bénin, les piles contenant du mercure (49%) l’orpaillage artisanal (22%) et les dépôts informels de déchets généraux (13%) sont les principales sources potentielles d’émission anthropique. Les régions actuellement les plus émettrices de mercure sont l’Asie et l’Afrique sub-saharienne avec respectivement 50% et 16,8% des émissions anthropiques. La formation du méthylmercure en milieu aquatique sous l’influence de divers facteurs et sa bioaccumulation sont principalement responsable des effets toxiques sur l’homme.Mots clés : Environnement, pollution, mercure, méthyl mercure, milieu aquatique, toxicité.English Title: Mercury: sources of emission, toxicity, contamination of aquatic environment and particularity of Benin republic.The massive mercury poisoning that has occurred in Japan and Iraq has led to a global awareness of the lethal power of this metal with the adoption by the international community of the Minamata Convention signed and ratified by Benin Republic. This article aims to synthesize knowledge on mercury through its emission sources, its toxicity with a focus on aquatic contamination in Benin. Thus, a review of scientific publications as well as the consultation of the archives of the Ministries of Environment and Health in Benin were carried out. Analyses of the various documents show that artisanal small-scale gold production and coal combustion are the main sources of mercury emission worldwide, with respectively 37.1% and 24.2% of anthropogenic emissions. Asia and sub-Saharan Africa would be the largest contributors to anthropogenic emissions with respectively 50% and 16.8%. In Benin, the main potential sources of anthropogenic emissions are mercury containing batteries (49%), followed by artisanal gold panning (22%) and informal deposits of general waste (13%). With regard to the behavior of mercury in the aquatic environment and its toxicity, it appears that methyl mercury is mainly responsible for toxic effects.Keywords: Environment, pollution, mercury, methyl mercury, aquatic environment, toxicity Bénin.

The present study aims to fill the data gap analysis in urban wastewaters characteristics in Benin and its statistical analysis. Physicochemical parameters such as pH, electrical conductivity (EC), Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD5), Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP) and UV Absorbance at 254 nm, were determined on domestic (greywater and blackwater) and industrial (hospital, pharmaceutical and commercial laundry) wastewater in Cotonou city. Analysis of variance showed a strong significant difference in the physico-chemistry of the various effluents. The pharmaceutical wastewater has the highest concentration of organic pollution (COD = 5,912 ± 1,026 mg/L, Abs.UV254 = 2.667 ± 0.327 cm−1). The organic load of blackwater is mainly in particulate and biodegradable form. Besides, the correlation study showed the limits of pH and EC as an indicator of organic load. Furthermore, the choice of COD or BOD5 as the main design parameter would be limited to blackwater treatment. Abs.UV254 was found to be the parameter having a strong relationship with other parameters of all effluents except blackwater. It then takes priority over COD for the treatment of greywater and industrial wastewater. For future wastewater treatment plant design, we recommend to consider Abs.UV254 as an important parameter.