Evaluation of different measurement techniques to investigate the clogging phenomenon in constructed wetlands

Constructed wetlands require a thorough understanding of system hydraulics for their correct design and efficient operation. As part of the treatment process, filter media will gradually become clogged due to factors related to influent characteristics, system designs and bed activities. The development of clogging can be detected by the appearance of water on the surface of the granular medium and it could reduce the treatment performance. Despite preventative and restorative strategies could be carried out to reduce the phenomenon, the clogging is a widespread operational problem and it has become increasingly important to identify practical methods for its measurement. Hydraulic behavior of constructed wetlands (CWs) could be investigated in situ by means of hydraulic conductivity (Ks) measurement, clog matter characterizations as well as hydrodynamic visualizations. The suitability of available measurement techniques, in terms of accuracy, repeatability as well as time and skill required, can vary depending on the substrate type, system design as well as clogging degree and distribution. In order to investigate to which extend some of the available measurement techniques are suitable to assess the spatial and-temporal evolution of clogging in pilot and full scale CWs, a monitoring campaign started few years ago. The full scale CW works as secondary wastewater (WW) treatment system of the Ikea®, located in the industrial district of Catania, Eastern Sicily, Italy. The pilot scale CW treats alternatively storm water from the Ikea® parking area and the sequential batch reactor WW produced from the retail store. Hydraulic conductivity measurements were carried out only in the horizontal subsurface CW beds by falling head tests specifically developed to detect high Ks values. In particular, different schemes and equations for the Lefranc’s test were compared employing also a new type of in situ permeameter cell. The implemented pervious permeameter used with a calibrated equation for estimating Ks (Licciardello et al., 2019) was the most suitable method to assess Ks to obtain an estimations of Ks that account also for the horizontal component of flow. In order to validate this new scheme – equation combination, other hydraulic measurement techniques were tested. In particular, tracer tests were carried out by pulse–injecting a sodium chloride solution into the inlet pipe of the hydraulic system and then measuring WW electric conductivity at different sites within the bed to assess the Ks variations. Moreover, pump test recovery data from a single observation well and cumulative drainage outflow data were analyzed to evaluate the contribute of these techniques to better understand the hydraulic behavior of CW beds.