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Study of vertical flow treatment wetlands by X-ray tomography

Abstract : Fouling and clogging are major concerns regarding Treatment Wetland (TW) operation and performances. Several techniques are currently used to assess these phenomena indirectly based on their relation with physical, chemical and biological parameters of TW (e.g. hydraulic conductivity, fouling material composition, and enzymatic activity respectively). Few of them focus on the geometrical structure of porous media, which is essential to understand the mechanisms governing solid filtration. This work presents the use of X-ray Computed Tomography (CT) as an imaging methodology to study French Vertical Flow Treatment Wetlands (VFTW). It is a non-destructive technique that produces 3D images of an object showing its inner structure at high resolution. Images from samples of French VFTW show the presence of three phases: voids, fouling material, and granular media (gravel). A methodology to extract, and scan the samples and segment the phases in the images is developed in this work. The use of state-of-the-art image processing algorithms to measure pore scale properties is demonstrated. The spatial representativity of pore scale measurements is assessed: (i) a 2D-Representative Elementary Volume (REV) of the volume fraction of the phases was determined. The assessment suggests that the sample diameter should be larger than 1.5 cm. To avoid significant shear stress during the sample extraction, the minimum recommended diameter is however 5 cm, (ii) pore scale properties were compared between samples extracted from different locations of a large region of a mature French VFTW, which was found to have homogeneous electrical conductivity by using a Frequency Domain Electromagnetic Method. Several similarities among the samples at the pore scale are highlighted. Finally X-ray CT was used to study the structural changes of porous media in French VFTW during the rest period and their influence on the fouling and clogging processes. Despite a limited evaporation rate (2 mm/d), a significant reduction of the deposit layer thickness that is formed above the filter has been observed (with values dropping by 12% to 34% of the initial values). Changes within the gravel filter were also noticed and account for the importance of the rest period to maintain a connected macropore network and to allow air to enter the microporosity of the fouling material in French VFTW
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Submitted on : Monday, July 12, 2021 - 4:28:14 PM
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  • HAL Id : tel-03157139, version 2

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German Dario Martinez Carvajal. Study of vertical flow treatment wetlands by X-ray tomography. Chemical and Process Engineering. Université de Lyon, 2020. English. ⟨NNT : 2020LYSE1018⟩. ⟨tel-03157139v2⟩

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