Characterization of constructed wetland substrates and evaluation of their hydraulic behavior

Green infrastructures, as constructed wetlands, play an important role in wastewater treatment and reuse particularly in small communities and agro-industrial activities. As part of the treatment process, filter media of constructed wetlands (CWs) will gradually become clogged due to factors related to influent characteristics, system design 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. The phenomenon is very variable in space, being generally most severe within the first few meters of the CW but, sometimes, it also occurs close to the outlet area. Hydraulic behavior of CWs can be investigated in situ by means of hydraulic conductivity (Ks) measurements, 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 the suitability of measurement techniques 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 was the most suitable method to obtain a Ks estimation that account also for the horizontal component of flow. In order to validate the proposed method, 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.