Caractérisation d'un Système Lagunaire en Zone Tropicale: Cas du lac Nokoué (Bénin)

This survey permitted to make hydrological balance of the lake Nokoué and to know quality physico-chemical of the waters of the lake in order to offer the basis elements for a future ecological modeling of the system. The results of hydrological balance show that the average of the flow of the soft waters received by the lake in period of high waters is 216m3/s essentially brought by the Ouémé stream, the longest stream of Benin that crosses the country from the north to the south. This value falls to 49m3/s in dry season with 86% of the flow brought by the Sô river in dry season. In dry season, the Sô at Ganvié has the most important rate of organic load (53% DBO). For the phosphorus in dry season, the highest rate is gotten at the level of strongest rate is gotten at the level of Ouémé stream at Totchè (42%). In the period of high waters (from August to October) a dilution effect of the contribution is observed. In term of nutrient loads brought by purification systems, the results obtained are 360kg/j-P and 840 kg/j - N for Cotonou and Calavi and 150 kg/j- P and 700 kg/j-N for Ganvié. The contributions of "Acadjas " give 2,3 - 2,9 kg/j P and 29 - 36 N kg/j. The main results obtained for the physico-chemical for characterization are presented as follows: * a very remarkable saline intrusion in dry season, Sô at Ganvié the value passes from 0g/L in period of high waters to 2,5g/L in dry season, * the average pH varies between 6,6 and 7,5 for the waters of the bottom and between 6,9 and 7,8 for the waters of surface of the lake. The pH varies very little between the bottom of the lake and its surface, * the values of nitrogen and phosphorus nutrients are extensively high than the limited values indicated by the system of classification by the Organization of Cooperation and Economic Development (OECD). The waters of the lake are therefore eutropheses. We notice a seasonal evolution of the system. According to the factors of control hyacinths of waters, it is concluded that nutrients control the development of hyacinths when the salinity doesn't inhibit the growth of hyacinths anymore; that means of July to January

Efficiency of arsenic oxidizing bacterial biofilms for arsenic contaminated drinking water treatment

In drinking water supplies, arsenic exists mostly as two inorganic forms, arsenite [As(III)] and arsenate [As(V)] which are toxic to living organisms . According to WHO recommendations, the drinking water standard was reduced from 50 to 10 µg/L and many regulatory agencies have recently accepted this new standard. Most of the existing treatment processes are effective only on arsenic anionic forms [As(V)] and not on neutral and mobile arsenic complexes. To overcome this lack of efficiency, a first oxidation step of As(III) form is necessary and is usually performed using strong oxidant or binding materials that are costly for small drinking water treatment units. An alternative to theses physico-chemical treatments is the biological treatment using As(III)-oxidising bacteria. Numerous autotrophic bacteria are able to oxidise arsenic. Among them, Thiomonas arsenivorans [4-6] is able to oxidise As(III) up to 100 mg As(III)/L and appears to be a good candidate for its known capacity to use As(III) as an energy source and carbon dioxide or carbonates as carbon source. An As(III)-oxidizing biological treatment pilot unit coupled to trapping units for As(V) removal at the outflow of the biological bioreactor was performed on site in order to study the strength of the biological process in real operating conditions. The bioreactor was previously inoculated with the autotrophic As(III)-oxidizing Thiomonas arsenivorans. Then, it has been intermittently fed with contaminated water from the drinking water well, at site temperature (15-17°C) and under downstream mode. As(III)-oxidizing biofilm development has been followed during the pilot functioning using CE-SSCP-16S (targeting the global community) and PCR-DGGE-aoxB (targeting As(III) oxidizers) fingerprinting techniques. Results showed a complete colonization of the mineral support (i.e. pozzolana) by indigenous bacteria of the groundwater to be treated. Moreover, the oxidation yield of the biological step was in the range of 54 to 100 % depending on the residence time (from 30 to 7 minutes) and the residual As concentration at the end of the complete treatment process (biological oxidation and trapping) was below 2 µg As/L. These results are thus very encouraging for an industrial application in regard to the strength and its absence of nutrients supply, except for the low amount of oxygen needed if it is not in sufficient concentration in the site water.

Efficacité de biofilms de bactéries As-oxydantes pour l'étape de traitement biologique d'eaux potabilisables arséniées

L'arsenic est un métalloïde toxique dont la présence, relativement fréquente, dans les eaux et les sols est liée soit au fond géochimique, soit aux activités humaines. En ce qui concerne les eaux destinées à la consommation, la législation impose une concentration maximale en arsenic de 10 µg.L-1. Les effets nocifs de l'arsenic sur la santé humaine rendent nécessaire le développement de technologies efficaces et peu couteuse pour éliminer cet élément des eaux potables, ainsi que dans les aquifères pollués et dans les effluents miniers (Wang et Zhao, 2009). Une unité de traitement biologique d'eaux potabilisable faiblement arséniée (As< 50µg/L), couplée à une unité de piégeage de l'As en sortie du bioréacteur, a été mise en œuvre sur un site réel afin d'étudier la robustesse du bioprocédé. Un bioréacteur contenant de la pouzzolane (matériau utilisé dans les traitements d'eaux) a été préalablement ensemencé par une souche bactérienne As(III) oxydante autotrophe (Thiomonas arsenivorans) (Battaglia-Brunet et al., 2002, Michon et al., 2010 ; Wan et al., 2010) puis alimenté par l'eau issue du forage à température ambiante (15-17°C) avec un fonctionnement discontinu (asservissement de l'alimentation du bioréacteur à la pompe du forage d'alimentation en eau). Le suivi du développement du biofilm As(III) oxydant au cours du traitement biologique a été réalisé par la recherche des gènes codant pour l'ARNr 16S (diversité bactérienne totale) et ceux codant pour une arsénite oxydase (aoxB) (diversité des bactéries As(III)-oxydantes). Ce suivi a montré une colonisation rapide et stable du support minéral par des bactéries endogènes de l'eau à traiter. Le rendement d'oxydation de l'étape d'oxydation biologique est compris entre 54 et 100 % avec des temps de séjour de 30 minutes à 7 minutes qui sont comparables à des temps de séjour de techniques classiques de traitement. Les concentrations résiduelles en As en sortie du procédé complet (oxydation biologique + piégeage) sont inférieures à 1 µg/L, et qui sont donc très encourageants pour une application industrielle

Arsenite oxidation and arsenate determination by the molybdene blue method

International audienceBased on the similarity in properties of arsenate and phosphate, the colorimetric method using the molybdene blue complex was tested in order to determine low As(V) concentration in waters. The influence of complex formation time, daylight, temperature and competitive anions (silicate and sulphate) upon complex formation was determined. Optimal complex formation was reached in 1 h at 209/1 8C and was slightly favoured when developed in daylight. The formation rate declined with decreasing reaction temperature and no influence of any of the competitive anions tested (at concentrations usually found in natural waters of granitic areas) was noted. The detection limit of this method was 20 mg As(V) l 1. This simple, fast and sensitive arsenic determination method is suitable for field analysis, especially for waters containing low levels of phosphate and organic matter. Through arsenate determination, this colorimetric method allowed the arsenite oxidation efficiency of five common industrial oxidants to be compared. H2O2 and MnO2(s) were not considered as effective oxidants as a high excess was necessary to ensure As(III) oxidation. NaOCl and KMnO4 were promising oxidants as they allowed complete arsenite oxidation with a small excess for NaOCl or even less than the electron stoichiometric ratio in the case of KMnO4. FeCl3 was the most effective oxidant among the reagents tested here

Méthodologie et résultats du diagnostic de l'eutrophisation du lac Nokoué (Bénin)

Cette thèse porte sur la méthodologie et les résultats du diagnostic de l eutrophisation du lac Nokoué au Bénin, en Afrique de l Ouest. Elle a traité principalement de la question de la prolifération des jacinthes d eau. La problématique de la jacinthe d eau, conséquence de l eutrophisation et la nécessité d une approche systémique faisant recours à la modélisation informatisée ont été relevées. Les paramètres physico chimiques de la qualité de l eau et des sédiments ont été suivis pendant deux ans (2006 et 2007). Les résultats ont confirmé l état eutrophe du lac Nokoué et ont mis en évidence la variabilité saisonnière de la qualité des eaux. La production des jacinthes d eau est régulée par la salinité avec un seuil limite de croissance moins élevé qu en zone tempérée (10 pour le Nokoué contre 15 en zone tempérée). Les résultats du modèle montrent que les apports nutritifs proviennent essentiellement de la rivière Sô (97% N et 89% P). La salinité a un effet dominant sur l indice intégré de croissance des jacinthes d eau (SII) dans toutes les stations suivies.This thesis focuses on the methodology and results of the diagnosis of eutrophication of Nokoue lake in Benin (West Africa). It dealt mainly with the question of water hyacinth infest. The problem of water hyacinth, a consequence of eutrophication and the need for a systemic approach by computer modeling have been reported. The chemical quality of water and sediment were monitored for two years (2006 and 2007). The results confirmed eutrophic status of Nokoue lake and showed the water quality changes with seasons. The production of water hyacinth is regulated by salinity with a threshold of growth lower than in temperate zone (10 for Nokoue against 15 in temperate zone). The model results show that nutrient inputs are mainly from the Sô River (97% N 89% P). Salinity has a dominant effect on the integrated index of water hyacinth growth (SII) in all followed stations.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF

Cell surface characterisation of Microcystis aeruginosa and Chlorella vulgaris

International audienceAccording to previous studies, Microcystis aeruginosa, a picocyanobacteria, is more sensitive to copper than the other phytoplanktonic species such as Chlorella vulgaris a chlorophycea. An experimental study was carried out to determine the cell wall characteristics of both species. The cell surface of M. aeruginosa presented a lower hydrophobic character and a more negative surface charge over a larger pH range than for C. vulgaris, according to hydrophobicity and zeta potentials measurements. By combining infrared spectroscopy data and potentiometric titrations, the total concentration of functional groups determined for M. aeruginosa (1.60 × 10−3 mol/g), as well as the number of carboxyl groups, was higher than for the C. vulgaris ones (0.22 × 10−3 mol/g). The differences in surface properties, as shown by chemical, physicochemical and spectroscopic measurements would justify the differences in sensitivities to copper ions up-take between the two specimens

Influence of aeration onto As trapping in ZVI/sand reactor

Arsenic toxicity and occurrence in the environment lead to the research of easy to handle and cheap water treatment process. Zero Valent Iron (ZVI) supports meet these requirements. ZVI trapping capacity is due to iron by-products, such as green rusts, lepidocrocite... According to physico-chemical conditions applied, the nature of by-products changes and arsenic trapping capacity is highly impacted. The aim of this work concerns the influence of aeration onto arsenic trapping in ZVI/sand columns. Four up-flow reactors filled with a ZVI/sand mixture were fed with synthetic water spiked with AsV. Two columns were aerated by air diffusers placed at the bottom while the two others were not aerated. Arsenic concentrations were monitored for three months in order to characterize arsenic sorption capacity of both configurations. A nearly complete removal was observed during the first five days into aerated system, whereas arsenic removal was stable, close to 50% during the first 20 days for the non aerated units. After this period an inversion of arsenic trapping capacity was observed, the non-aerated columns showed a higher arsenic removal than the aerated ones. Finally, when all columns reach their saturation, highest As trapping capacities were obtained in the non-aerated systems. According to arsenic monitoring, these retention capacities were estimated at 220 and 140 mg(As)/g(Fe) for non-aerated and aerated columns respectively. Solid support samples were collected along the reactor and four different layers were distinguished. After acid mineralization, analysis of total arsenic highlighted different repartitions inside the reactors: a homogeneous concentration in the non-aerated systems and a decreasing concentration form inlet to outlet for the aerated pilot units. In order to characterize As sorption into ZVI/sand columns, breakthrough curves were modelized using Thomas model. The determined KTh constants were different for aerated and non-aerated columns, which confirmed that the affinity of arsenic for solid support was different in the two systems. Oxygen measurement showed that synthetic water was close to saturation for aerated systems while it increased from 0.5 to 8 within 70 days in the non-aerated units. According to iron corrosion mechanisms, aeration induced a fast ZVI iron by-products generation, and thus a higher arsenic trapping capacity at the beginning of the experiment. Arsenic sorption into non-aerated reactors was lower but lasted longer, due to the regular ZVI-by-products generation. The lower trapping capacity of aerated systems could be due to a conversion over time of the early-created byproducts to less reactive forms

Distribution of As trapping along a ZVI/sand bed reactor

International audienceZero-valent iron (ZVI) based reactors are widely used to remove arsenic from water and have been extensively studied, though typically by considering just the inlet and outlet arsenic concentrations. This paper presents the arsenic distribution inside an aerated bed reactor filled with a ZVI/sand support. The removal performance was evaluated through both the classical monitoring of arsenic concentrations and an analysis of the support upon completion of the experiment. Both types of analyses were performed not only at the column inlet and outlet but also at various intermediate sampling points in order to highlight the different behavior encountered along the column. The Thomas model was applied to simulate the breakthrough curves and determine sorption constants. These distinct approaches yielded similar results: while the iron distribution remained homogeneous from one approach to the next, the arsenic removal was five to six times higher at the inlet to the bed reactor than in the final ZVI/sand layer, reaching 170mg(As)g-1(Fe). The influence of residence time was also studied, revealing that a higher arsenic load in the column led to greater retention within the first column layers

Influence of phosphonic acids and EDTA on bacterial copper toxicity

International audienceThe influence of hydroxyethane-1,1′diphosphonic acid (HEDP) and aminotris(methylene) phosphonic acid (ATMP) on bacterial copper toxicity and comparisons with EDTA was studied. Two tests were used throughout the study, the normalised Microtox test and an assay (I24) based on Escherichia coli inhibition. The concentration range studied was mainly near the ones that are nowadays detected in the environment: 10-8-10-6M. A chemical approach was also performed in order to determine the speciation of the inhibitory copper forms in the working conditions used. Ligands lower copper toxicity after 10-6M, but protection by HEDP was lower than that of both EDTA and ATMP. For [ligand]/[copper] ratios close to 0.5 (with [ligand] > 10-6M), copper resistance increased in both tests in the presence of EDTA and ATMP but not in the presence of HEDP. For [ligand]/[copper] ratios close to 0.1 (with [ligand] = 10-7M) the presence of ATMP and EDTA seems to increase E. coli sensitivity to free copper