No abstract

French Vertical Flow (VF) treatment wetlands receive raw wastewater and provide simultaneous solids/sludge and wastewater treatment. This feature among others explains why French VF treatment wetlands have become so popular for wastewater treatment of small communities (less 4,000 people equivalent) in France. For proper solids/sludge handling, the treatment wetland must be designed adequatly and specific operational and envrionmental conditions must be maintained. When these conditions are not met, accumulation of biosolids may lead to clogging. Filtration and biological treatment in french VF Treatment Wetlands are governed by mechanisms at the pore-scale. Therefore, they must be better understand to predict reliably biosolid accumulation and clogging. X-ray Computed Tomography (Xray-CT) is a promising technique to characterize in detail the morphology of the filtering media in treatment wetlands. In order to set a solid basis for the use of Xray-CT, the spatial representativity of measurements must be assessed. This issue is addressed in this study by successively analyzing spatial properties at the filter scale using Frequency Domain Electromagnetic Measurements (FDEMs), and at the pore scale using Xray-CT. FDEM was used to obtain a map of the electric conductivity at the surface

Clogging constitutes a major operational issue for treatment wetlands. The rest period is a key feature of French Vertical Flow (VF) treatment wetlands and serves to mitigate clogging. An ex-situ drying experiment was performed to mimic the rest period and record structural changes in the porous media using X-ray Computed Tomography (CT). Samples containing the deposit and gravel layers of a first stage French VF treatment wetland were extracted and left to dry in a control environment. Based on CT scans, three phases were identified (voids, biosolids, and gravels). The impact of the rest period was assessed by means of different pore-scale variables. Ultimately, the volume of biosolids had reduced to 58% of its initial value, the deposit layer thickness dropped to 68% of its initial value, and the void/biosolid specific surface area ratio increased from a minimum value of 1.1 to a maximum of 4.2. Cracks greater than 3 mm developed at the uppermost part of the deposit layer, while, in the gravel layer, the rise in void volume corresponds to pores smaller than 2 mm in diameter. Lastly, the air-filled microporosity is estimated to have increased by 0.11 v/v.