Date of Award
Doctor of Philosophy (PhD)
Treatment of municipal and rural wastewater using subsurface flow constructed wetlands (SFCW's) is growing in popularity. These systems often consist of an excavated cell containing 2-3 in. rock with the wastewater level maintained below the media surface. A SFCW is a natural form of wastewater treatment which removes organics and nutrients based on the attached growth of microorganisms on crushed rocks where biological reactions take place. The currently accepted design procedure is based on the first order plug flow model, although tracer studies have shown that flow conditions are not well represented by the plug flow condition. This design rationale has led to the construction of several SFCW's with high aspect ratios (L:W) to ensure plug flow. However, many of these systems have experienced water surfacing and channeling failure due to increased hydraulic resistance in a long narrow cell. To overcome this problem, new systems and modified older systems are now constructed with lower aspect ratios resulting in a decreased resistance to flow, preventing the wastewater from surfacing or channeling. However, lowering the aspect ratio results in an increase in dispersion which is neglected in the plug flow model. Therefore, plug flow is not ensured and alternative models such as a solution to the connective-dispersive equation describing the partially mixed flow condition should be considered. However, few tracer studies have been performed in SFCW's, and few estimates of the dispersion parameter are available in the literature. Therefore, the focus of this research was to develop a relationship between the longitudinal dispersion number and the interstitial velocity as a function of the system's aspect ratio and flow rate. The procedure involved designing and constructing a bench-scale SFCW model, and determining dispersion numbers for varying aspect ratio and flow rate arrangements by performing tracer studies in the bench-scale model. Results indicated that as aspect ratio decreases, dispersion number increases and as flow rate increases, dispersion number increases. Also, the dispersion number decreases exponentially with increasing hydraulic residence time when considering aspect ratios from 6:1 to 2:1. The range of dispersion numbers found in this study were from 0.107 to 0.345.
Cothren, Gianna M., "Longitudinal Dispersion and Interstitial Velocity Relationship in Subsurface Flow Constructed Wetlands." (1996). LSU Historical Dissertations and Theses. 6241.