Candela, L.; Vadillo, I.; Aagaard, P.; Bedbur, E.; Trevisan, M; Vanclooster, M.; Viotti, P. & López-Geta, J.A. (Eds.), WAter POllution in natural POrous media at different scales. Assessment of fate, impact and indicators. WAPO 2 © Instituto Geologico y Minero de España, Madrid, 2007. ISBN: 978-84-7840-676-0 329 ANALYSIS OF HYDRODYNAMIC CHARACTERISTICS OF A HORIZONTAL SUBSURFACE FLOW CONSTRUCTED WETLAND A. Albuquerque (1) and R. Bandeiras (1) (1) CECUBI, Department of Civil Engineering and Architecture, University of Beira Interior, Edificio 2 das Engenharias, Calcada do Lameiro, 6200-358 Covilha (Portugal) Email: ajca@ubi.pt ABSTRACT Horizontal subsurface flow (HSSF) constructed wetlands are widely used for wastewater treatment but there’s still a lack of information on flow characteristics changes throughout the porous bed over time. The continuous growth of roots, the adsorption, sedimentation and precipitation of wastewater compounds and the biofilm release over operation time leads to the development of stagnated areas, the appearance of hydraulic short-circuiting and variations in dispersion conditions, which affect substrate distribution throughout the bed and, therefore, may worsen the overall performance of the system. In order to enable a better understanding of the transport mechanisms throughout the bed, two series of tracer tests were performed in a laboratory HSSF system at the hydraulic loading of 4.7x10 -3 m 3 m -2 h -1 . The first series was executed with the bed without vegetation and the second with the bed already colonized with Phragmites australis after an operation period of eight months at an average organic loading of 12 g m -2 d -1 COD. The magnitude of longitudinal dispersion was estimated both by a curve- fitting technique using the non-linear least square optimization method (LSOM) and using the moment method (MOM) over the advection-dispersion-reaction equation. The flow regime and the extension of dead volumes were estimated by the same curve-fitting technique over the multiple tanks in series model (MTS). The results showed that the development of plants and biomass did not contribute for a significant amount of longitudinal dispersion in the overall media length. However, the dispersion was very strong in the inlet section, where the dead volumes increased approximately 65% within the colonized bed. The flow regime may be considered as plug flow throughout the overall length and the tracer retention was associated with dispersion and internal recirculation and, additionally, for Series II it was associated with the occurrence of clusters of biomass and roots. The MTS better explain the tracer results for the overall media length. The MOM seems to be inadequate for estimating Pe leading to underestimated values when compared with the ones calculated by LSOM. Keywords: Hydrodynamic characteristics, HSSF constructed wetland, dispersion