Clogging in constructed wetlands: Indirect estimation of medium porosity by analysis of ground-penetrating radar images

Abstract

Identification of the degree of porous medium clogging in constructed wetlands (CWs) is a complex procedure because bed obstruction is a phenomenon that occurs in the subsurface and involves several factors. Ideal methods must allow for subsurface analysis and be non-invasive, what is the case of ground penetrating radar (GPR or georadar) used for soil characterization. Thus, this work aimed to evaluate the use of GPR for clogging characterization of two full-scale horizontal subsurface flow constructed wetlands (HSSF-CWs), one unit planted with cattail (Typha latifolia) and the other unit with no vegetation. Both units received municipal sewage (50 p.e. each) previously treated in an anaerobic reactor, had been in operation for seven years (at the time the GPR was used) and showed signs of heavy clogging, leading to surface flow. In order to produce a standard for GPR image (created by the response of the reflected wave in the passage through different media) identification and association of colors with the environmental conditions inside the medium (cleaner or more obstructed condition), a clean granular rock filter (in operation for only one month), similar to the unplanted unit, was used. Equations developed for indirectly estimating the porosity, based on the RGB (red, green and blue) color scale, indicated that the methodology was in agreement with the visual conditions of surface flow occurrence in the HSSF-CW, showing to be a suitable non-invasive method to characterize the advancement of clogging in CWs.