Jensen, Lenore. 2000. The effects of plant abundance on a pilot scale treatment wetland. M.S.
The City of Stanwood in Snohomish County, WA, USA, is currently operating an experimental constructed wetland system to treat primary municipal effluent. Since the wastewater treatment plant discharges into the Stillaguamish River, there is concern about meeting NPDES limits. In addition, a limitation on ammonium (NH4-N) has been proposed. Water quality monitoring has been collaboratively conducted at this wetland since June 1998 in three separate studies. The first examined the impact of loading, temperature, and wetland type (shallow vs. deep) on system performance. The second examined the impact of plant species composition in the deep wetland on pollutant removal efficiency. Since earlier research had shown tremendous variation in percent plant cover in the deep water sections, the present study has been specifically designed to examine its relevance. This is frequently cited as an important design parameter that can be controlled by constructed wetlands operators.
In November 1999 the influent flow rates were equalized and four free water surface wetland cells began receiving similar loading rates. Temperature (°C), dissolved oxygen (DO), pH, five-day biochemical oxygen demand (BOD5), total suspended solids (TSS), total phosphorus (TP), and NH4-N were monitored at least monthly. Vegetative sampling, along with a conservative tracer study to measure HRT, were conducted in August 2000. Although the cells had substantially different vegetation coverage, the only significant differences in between-cell effluent concentrations were found for summertime TP and annual + summertime NH4-N. These differences were not attributable to plant cover or to HRT, and plant cover did not alter HRT.
These findings are important because the City expended a good deal of effort in vegetative planning, especially with respect to species composition, yet plant species and plant cover were relatively unimportant factors in pollutant removal. The results of this study strongly suggest that in humid temperate regions, loading rate, seasonal cycles in temperature, and wetland type may have very important impacts on system performance, while wetland plant species and percent plant cover appear to be relatively insignificant. This latter finding is especially relevant from a design and operation/maintenance perspective because while plant species and cover can be manipulated, this can be very labor intensive and therefore costly. These results suggest it is not necessary to maintain a specific type of plant cover in constructed wetlands.
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