Assessment of X-ray Computed Tomography to characterize filtering media from Vertical Flow Treatment Wetlands at the pore scale
Résumé
Computed Tomography is a non-destructive technique often used in earth sciences for the description of porous media at the pore scale. This paper shows the feasibility of this technique to obtain 3D descriptions of filtering media in Vertical Flow Treatment Wetlands (VFTW). Three different samples from two full-scale VFTW were scanned. The samples vary in moisture content and gravel size distribution. The 3D images show three characteristic phases of unsaturated media: voids, fouling material and gravel. The gray contrast level is good enough to perform phase segmentation successfully using region growing algorithms. In this study the results from segmentation are used (i) to compute profiles of phase volume fraction and specific surface at high resolution, (ii) to observe 3D distribution of isolated elements, (iii) and to draw the void's skeleton and to perform a percolation pathway study. This method highlights the presence of a transition zone between the deposit cake and the dense gravel layer. In this zone, mechanical interactions between gravels and filtered solids tend to promote a heterogeneous layer of gravel, fouling material and open porosity. The presence of isolated gravels in the deposit layer is clearly evidenced. The effect of drying to enhance the contrast between phases has been analyzed for one sample by a direct comparison of images obtained before and after drying. The resulting opening of the void phase tends to increase significantly the void-fouling material specific surface and the number and size of percolating pathways computed as the skeleton of the void phase. Finally, a first analysis of filtration processes is proposed. It consists in analyzing the percolation pathways for a class of void size by applying the distance map and skeleton concepts to the void phase. © 2018 Elsevier B.V.
Mots clés
Solvents
Wetlands
Constructed wetlands
Mechanical interactions
Porous materials
Nondestructive examination
Musculoskeletal system
Image enhancement
Computerized tomography
algorithm
filtration
constructed wetland
Non-destructive technique
Phase distribution
Phase volume fraction
Region growing
Region growing algorithm
Gravel
fouling
moisture content
percolation
porous medium
size distribution
water treatment
X-ray analysis
article
case report
clinical article
feasibility study
human
moisture
porosity
skeleton
x-ray computed tomography