Key influences on hydraulic efficiency in treatment wetlands

摘要

Too much of a good thing can become a problem. This is certainly the case with nutrients in surface waters. Excess nutrients are a concern in streams and lakes. While there are direct health risks related to drinking water contamination among vulnerable populations including infants, harmful algal blooms are a more prevalent concern since they manifest themselves at enrichment levels well below accepted drinking water standards. Half of the lakes in the United States have elevated nutrient levels, a condition that can ultimately lead to oxygen depletion. This problem is exported across state and national boundaries into coastal waters. Agricultural nutrient discharges are particularly difficult to address because, unlike end-of-pipe discharges, fertilizer runoff is hard to capture and treat in a cost effective manner.;Appropriate technologies are needed that promote agricultural production through the sustainable management of natural resources. Treatment wetlands are a low-tech alternative to conventional water treatment. Constructed wetlands provide passive treatment of nutrient enriched runoff and other diffuse non-point sources of contamination through nutrient uptake, absorption, or chemical reduction. Hydraulic inefficiencies can substantially limit nutrient reductions when stagnant zones and preferential flow paths exist that reduce contact time. Optimally configured wetlands cost less and perform better. Unfortunately, it is not clear what constitutes an optimal configuration.;Many factors, including shape, depth, and botanical structure, influence hydraulic efficiency. The various factors also influence each other, which makes it difficult to ascribe an effect to any one particular factor. Conventional investigative methods using controlled experiments focusing on a response to a single factor cannot tell the whole story. A more comprehensive approach is described here.;Scaled models were used to investigate treatment wetland hydraulics. Scaling shortens the time required for a given test, making a sufficiently large number of observations more practical. Modifications were be made to the overall shape and form to achieve variation from one observation to the next. The study considered effects initially from twenty-five parameters related to the model configuration, hydraulic loading, and placement of vegetation. Eventually seven key parameters were identified. The parameters are related to (1) the alignment of the inlet and outlet, (2) a shape factor, (3) the vegetation coverage, (4) the number of wetland cells, (5) the nominal residence time, (6) stem density, and (7) the depth of water. A mathematical expression was derived relating the seven parameters to hydraulic performance.;Practitioners can apply the resulting equation to evaluate wetland designs and make informed decisions with regard to tradeoffs involving hydraulic efficiency. Until now, there was no way to evaluate wetland designs in terms of hydraulic performance without invoking highly involved numerical simulations running on commercial software or else building and testing a design, at which point modifications are costly and limited. Ultimately, this tool promotes the adoption of constructed wetlands as a management practice by informing the design process with respect to hydraulic efficiency.