Investigating the hydraulic performance of fullscale treatment wetlands using electrical resistivity tomography

Horizontal subsurface flow constructed wetlands (HSFCWs) have been widely used for tertiary treatment of wastewater in the United Kingdom (UK). One of the main objectives for UK’s water industry is achieving a reduction in maintenance costs of HSFCWs by reducing frequency and cost of clogging remedial measures. However, inability of conventional clogging assessment methods to provide reliable subsurface visualisation of the HSFCWs limits understanding of clogging profiles across treatment units, hindering proactive maintenance and remedial interventions. Without trustworthy information about subsurface conditions of HSFCWs, clogging remedial measures cannot be optimised to provide targeted, proactive, and cost-effective maintenance. In this study, electric resistivity tomography (ERT) has been applied to identify its effectiveness to provide reliable visualisation of the subsurface conditions of HSFCWs. On this project, two HSFCWs with different clogging levels were tested using ERT. Anomalies in subsurface distribution of water and profiles of clog matter accumulation across the HSFCWs were identified by changes in electric resistance of the media. The output was in form of cross sections showing variation in resistance across the media with different colours. The visual representation of subsurface conditions covered the full depth as well as the length of HSFCWs. The subsurface conditions identified in ERT results were in line with those observed by conventional clogging measurement methods, including substrate permeability measurement and tracer test, which provides preliminary validation of the ERT methodology. A recently refurbished HSFCW was found to have negligible level of clogging and subsurface visualisation was depicted by consistent shades showing uniform electric resistivity values (between 20 to 300 ohm.m) across the HSFCW. Contrarily, a mature HSFCW with many years of operation and no recent remedial interventions resulted in significant colour variation across the sections, indicating uneven electric resistance across the HSFCW (between 20 to 4000 ohm.m) and the ability of the ERT methodology to infer clogging levels. Considerably higher resistivity values and their chaotic variations across the subsurface indicated the distribution of wastewater and clog matter. To Identify the effectiveness of ERT to visualise subsurface of wetland systems, two different ERT arrays (electrode configurations) were used: Dipole-Dipole characterised by better horizontal resolution and effective depth coverage at the ends of ERT line and Wenner Array characterised by better vertical resolution and less susceptibility to noise. Both arrays were successful in identifying variations in local clogging levels in the HSFCWs, with the Dipole-Dipole array results found to be more relatable with actual site conditions. This work has evidenced the potential of ERT to profile clogging in HSFCWs, providing a tool for implementing targeted asset maintenance and reduced operational costs. Further improvement in the ERT methodology is recommended by testing the impact of smaller probe spacing, controlled water levels in HSFCWs, and increased observations of the same systems at different operation ages.