INVESTIGATION OF HYDRAULIC CHARACTERISTICS AND ALTERNATIVE MODEL DEVELOPMENT OF SUBSURFACE FLOW CONSTRUCTED WETLANDS
Abstract:The currently practiced design of subsurface flow constructed wetlands (SFCW's) for wastewater and stormwater treatment is based on the plug flow assumption. Since SFCWs are now commonly designed with low aspect ratios to circumvent surfacing problems, plug flow is not ensured; therefore, alternative models which more accurately describe the flow should be considered. However, to solve the alternative models, a value for dispersion is required. The fundamental variables affecting the dispersion number are the interstitial velocity, which is dependent upon flow rate, porosity, and cross-sectional area, and the pore geometry, which is dependent on the media characteristics such as the average grain diameter and permeability. In addition to these fundamental variables, the dispersion number will also fluctuate with a series of uncontrollable variables specific to a particular SFCW situation. For example, differences in plant root growth and die off, change in hydraulic conductivity with time, inconsistencies in construction, unsteady-state flow, and animal habitation are all variables which cause uncertainty in the dispersion number. Nonetheless, if new advances are to be made in the design of SFCWs, an estimate of dispersion will be required in order to model the system more accurately. The intention of this research is to extend the results of an empirical relationship between the interstitial velocity as a function of flow rate and aspect ratio and the dispersion number developed using a bench-scale model to the field scale application. Additionally, a hydraulic characteristic evaluation, including a water balance of the inflows and outflows, precipitation, and evapotranspiration, and the hydraulic residence time and estimation of the dispersion coefficient will be performed.
Document Type: Research Article
Publication date: January 1, 2000
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