Épuration naturelle : de la rivière à la zone humide de rejet

Abstract : The present PhD work was carried out within the project EPEC (Epuration en Eau Courante) funded by an ANR program, ECOTECH, in order to meet the requirements of Water Framework Directive for small streams, in particular in rural areas where domestic wastewater could be directly discharged by reason of lack of sewerage network and contribute to water quality degradation. Two study directions have been taken: i) the first aimed to study natural purification in stream systems and find out the way to improve water quality, and 2) the second concerned the reduction of the impact of wastewater treatment plants (WWTP) discharge to receiving water bodies by installation of a free-surface constructed wetland between them. Three study scales were investigated within two rural streams of Lorraine, Brénon and St-Oger. At stream scale, characterization of water quality along its course allowed us to distinguish some segments where occurred naturel purification processes. The second study scale was on relevant stream sections presenting interesting hydromorphologic features. These sections were located at the downstream of urban areas and present a succession of rectified and naturel segments. The response of naturel sections to domestic pollutants was different for the two streams. The Brénon section length of about 6 km was efficient for organic matter, ammonium nitrogen and nitrates removal. Concerning the St-Oger stream, the pollutants were less influenced in the natural section long of only 0.5 km. The last study scale focused on the hyporheic zone where system function depends on hydromorphologic features of the stream, composition of streambed, especially its porosity, and hydrologic conditions which depends on climate. According to analysis on hyporheic waters sampled at -30 and -50 cm for Brénon and -20 and -30 cm for St-Oger, four functional zones were distinguished in relation with dissolved oxygen availability and possible water exchange between hyporheic zone and surface water: (1) aerobic zones at high hyporheic exchange showing contribution to organic matter degradation and oxidation of ammonium nitrogen; (2) Anoxic zones with less hyporheic exchange characterized by fast dissolved oxygen depletion by aerobic microbial metabolism and reduction of nitrates; (3) Anoxic zones with low hyporheic exchange characterized by accumulation of salts in deep layers and reduction of nitrates and sulfates and (4) “closed” zones characterized by clogged spaces or very low hydraulic conductivity. These functions could be partially reproduced in laboratory within a porous bed reactor simulating an hyporheic zone. Free-surface wetlands are spaces constructed between the discharge point of the WWTP and the receiving watercourse, here small streams in rural areas, with the aim to finish the waste water treatment. The wetlands had shown high capacity to remove nitrates and phosphates in summer periods. However a production of dissolved organic carbon was noticed and results from plant decomposition (reed, duckweed, algae, etc.). Algae contributed to high oxygen production through photosynthesis in spring. This production progressively decreased with the proliferation of duckweed on the water surface. Two biological tests on sediment's potentiality for denitrification and methane production were carried out at laboratory scale in order to corroborate the field observations
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Yuhai Zhang. Épuration naturelle : de la rivière à la zone humide de rejet. Autre. Université de Lorraine, 2014. Français. ⟨NNT : 2014LORR0302⟩. ⟨tel-01751535⟩

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