Phosphorus Removal Performance of a Large-Scale Constructed Treatment Wetland Receiving Eutrophic Lake Water

Eutrophication continues to impact watersheds and their receiving water bodies. One approach to mitigate this problem is to use constructed treatment wetlands. Our objectives were to determine long-term phosphorus (P) removal by a large-scale constructed treatment wetland (the marsh flow-way at Lake Apopka, Florida, USA) that treats lake water and to quantify the monetary costs for performance. The marsh flow-way treated substantial amounts of lake water (30myr−1, which is about 30% of the lake’s volume on an annual basis). Associated with this, P was removed at an average rate of 0.85gm−2yr−1 (2.6 metric tons yr−1). The marsh flow-way removed mostly particulate P, while it released dissolved P fractions (mostly during the first few years of operation; thereafter, release was negligible). The long-term first-order removal rate constant (k) for P averaged 27myr−1. Phosphorus removal performance varied seasonally, with greater removal during cool periods (September–May) and poor removal during warm periods (June–August). Incurred annual operation and maintenance (O&M) costs averaged $455,000 (2012$), which was equivalent to $1,648hayr−1 or $177 per kilogram of P removed. We also calculated costs for a 25-year project life cycle, and compared the incurred and the 25-year costs to other systems that illustrated the marsh flow-way was cost competitive. Both P removal and costs were useful metrics in helping us determine operational and management changes. This resulted in a seasonal management strategy that contributed to increased P removal and a reduction in O&M, thereby increasing cost effectiveness. In addition to costs, treatment wetlands provide benefits that include a range of ecosystem services. We challenge ourselves and other treatment wetland managers to adopt both a cost and benefit approach to assessing system performance.