The effect of the scale of horizontal subsurface flow constructed wetlands on flow and transport parameters

Abstract

Horizontal subsurface flow constructed wetlands have proven their efficiency in treating wastewater and removing the pollutants of concern. Treatment efficiency depends on the wastewater residence time, which is a function of the hydraulic loading and the physical conditions of the constructed filter system, which can be described with effective parameters such as: hydraulic conductivity, porosity, dispersivity etc. Because spatial variability is often scale dependent, these effective parameters may be affected by the scale of the system being studied. In this paper the results of tracer experiments in constructed filters using saturated horizontal flow at three scales (small and medium lab scales and full-scale system) using the same filter media is reported. Light-weight aggregate (filter media termed Filtralite-P) was used at all scales. Increasing the scale was associated with increasing dispersivity, meanwhile hydraulic conductivity experienced dramatic reduction and variation by increasing the examined scale. Observed changes in the hydraulic parameters indicate that heterogeneity at different scales should be taken into account when the performance of LWA filters are evaluated from small-scale experiments.

Design methodology accounting for the effects of porous medium heterogeneity on hydraulic residence time and biodegradation in horizontal subsurface flow constructed wetlands

“…Moreover, the deviation from the ideal, symmetric case (in other words, the skewness of the BTC) increases with the distance from the inlet. This indicates that the effect of heterogeneity is scale-dependent, as already known theoretically (e.g., Dagan, 1984), in aquifers (e.g., Barry and Sposito, 1990) and from the experimental findings of Suliman et al (2005).…”

“…The low removal efficiency has been attributed to several reasons, including incomplete understanding of the purification mechanisms (Haberl et al, 2003;Imfeld et al, 2009) and the nonhomogeneous distribution of the porous substrate. This latter leads to poor hydrodynamic behavior of the system, which results from a broad distribution of the hydraulic residence time (HRT) and associated preferential flow paths (Suliman et al, 2005;Knowles et al, 2008;Mena et al, 2008;Ascuntar-Rios et al, 2009). This aspect appears to be critical in particular for HSCWs where the treatment efficiency decreases significantly compared with vertical systems (Matamoros et al, 2007b(Matamoros et al, , 2008.…”

“…Tracer experiments to study the factors controlling the porous medium heterogeneity in HFCWs were conducted by Suliman et al (2005). They reported that hydrodynamics in HSCWs becomes more complex as the scale of the system increases, behavior that is also found in natural aquifers (Gelhar et al, 1992).…”

“…As a consequence, traditional design methods may not be suitable to guarantee high water quality levels and new and more robust design tools are sought (Kadlec, 1997). While the connection between the heterogeneous distribution of the hydraulic conductivity, HRTD and removal efficiency is now widely recognized (Marsili-Libelli and Checchi, 2005;Suliman et al, 2005;Wörman and Kronnäs, 2005), it is not known how the design process can be improved to take it into account, and how the negative or positive effects of heterogeneity in working systems can be controlled. Numerical models have been extensively used in numerous applications in the field of the environmental sciences and engineering (such as hydrogeology, soil science, petroleum engineering, etc.)…”

Organic xenobiotics removal in constructed wetlands, with emphasis on the importance of the support matrix

“…Recently, despite the slightly increased cost of production, processed clay materials have also become popular choices for CWS substrates and in any case clay-based materials proved to be an adequate medium for a good performance of CWS [42,46,47,83,84,126,165,[199][200][201][202][203][204][205][206].…”