Centripetal filtration of groundwater to improve the lifetime of an MgO recycled refractory filter: observations and technical challenges.

In the context of improving permeable reactive barrier (PRB) filters, axial and a centripetal column tests were performed to compare their evolution in terms of chemical and hydraulic performances. For both tests, the MgO reactive media, made of crushed (< 10 mm) spent MgO-C refractory bricks was used to treat water contaminated with Co and Ni by raising the pH and promoting hydroxide precipitation. As opposed to the traditional cylindrical axial configuration, the centripetal column consists of an annulus of reactive media through which the water flows from the outer radius towards the inner radius. Under similar conditions (total reactive mass, porosity), the centripetal column is expected to delay the breakthrough of contaminants because of its higher cross-section and lower flow speeds at the entrance of the media. However, as we found in this study, the design of a granular radial filter poses several technical problems. Indeed, a breakthrough of the contaminants, accompanied by a decline in pH, was observed much sooner in the centripetal (100 pv) than in the axial (375 pv) filter. This lower performance was deemed to be due to a hydraulic shortcut and was supported by the results of a tracer test (average renewal volume much lower (199 ml) than the theoretical one (7530 ml)) as well as the observation of preferential clogging upon dismounting the radial filter. While the design of a filter that induces a purely radial flow still poses a technical challenge, this study contributes to advance the knowledge for centripetal radial filtration of groundwater in PRBs.