European Water 58: 179-184, 2017. © 2017 E.W. Publications Evaluation of clogging in HSF pilot-scale CWs using tracer experiments G.D. Gikas 1* , V.A. Papaevangelou 1 , K. Moutsopoulos 1 and V.A. Tsihrintzis 2 1 Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece 2 Centre for the Assessment of Natural Hazards and Proactive Planning & Laboratory of Reclamation Works and Water Resources Management, School of Rural and Surveying Engineering, National Technical University of Athens, Athens, Greece * e-mail: ggkikas@env.duth.gr Abstract: The worst operational problem that may occur during the operation of horizontal subsurface flow (HSF) constructed wetlands (CWs) is the clogging of the porous media. Clogging phenomena cause hydraulic malfunction, and therefore, may lead to inadequate performance of HSF-CWs, and finally, to the reduction of their lifetime expectancy. The main objective of the study was the investigation of clogging in HSF-CWs pilot-scale systems, and its influence on the hydraulics of these systems. Additionally, the study also aims at the evaluation of the sustainable operation of HSF-CWs after their long-term use. One HSF pilot-scale CW was used. The experiment was conducted using an impulse tracer test using KBr, which provided residence time distribution (RTD) curves, and generally, information regarding the effect of clogging on flow dynamics (possible short-circuiting, dead volumes, uneven distribution). For the conduction of the experiment, the conservative tracer was introduced at the inlet through the influent distribution system and the monitoring was performed at various points along the wetland bed (i.e., 1/3 and 2/3 of the length, and the outlet). Equations are used to compute, based on the tracer test data, the mean actual HRT, the tracer mass recovery, the normalized variance and the hydraulic efficiency. The results indicate that the unit operated smoothly without serious clogging phenomena. Generally, this system has shown a sustainable and successful long-term operation. The presented method is an effective method to evaluate performance of horizontal subsurface flow constructed wetlands in terms of clogging. Key words: Constructed wetlands, Horizontal subsurface flow, Hydraulics, Clogging, Impulse tracer test, Potassium bromide 1. INTRODUCTION Constructed wetlands (CWs) are vastly being used over the last decades throughout the world for the treatment of several wastewaters and the removal of various pollutants, such as nutrients, heavy metals, pesticides, organic substances, etc. (Wu et al., 2015; Papaevangelou et al., 2016a, 2017a, 2017b). This green technology offers many advantages against conventional treatment systems, such as easy and trouble-free operation, low maintenance and operational cost, tolerance to high flow and load variability, and use of natural energy sources (Kadlec and Wallace, 2009; Gkika et al., 2014). Horizontal subsurface flow (HSF) represent one of the most common types of CWs employed throughout the world. In HSF CWs, wastewater flows beneath the surface of the porous media in a fully saturated flow pattern (Kadlec and Wallace, 2009). The worst operational problem that may occur during the operation of HSF-CWs is the clogging of the porous media (Pedescoll et al., 2011; de la Varga et al., 2013). As wastewater passes through the system, the removal of the pollutants occurs through several interactions between sediments, substrate, micro-organisms, litter, plants, atmosphere and the wastewater (Nivala et al., 2012; Gikas and Tsihrintzis, 2010; Kadlec and Wallace, 2009; Tsihrintzis and Gikas, 2010). Several mechanisms, including biological, physical and chemical develop and eventually lead to the saturation and the gradual clogging of the porous media (Pedescoll et al., 2011; Nivala et al., 2012; Pozo-Morales et al., 2013). Clogging phenomena cause hydraulic malfunction, and therefore, may lead to inadequate performance of HSF CWs and to the reduction of their lifetime expectancy (Caselles-Osorio et al., 2007; Pedescoll et al., 2011; Nivala et al., 2012; de la Varga et al., 2013).