Performance Data from Model Constructed Wetlands for Wastewater Treatment

A study has been initiated to contribute to the development of process-based design approaches for constructed wetlands in cold climates. Eight bench-scale vegetated sub-surface bed wetland cells were placed in the climate-controlled environment of Montana State University's Plant Growth Center. Three each were planted with native cattail and bulrush, while two cells remained unplanted as a control treatment. Since April 1996 the cells have received a stream of synthetic wastewater at a rate of 30 ml/min, resulting in a hydraulic residence time of approximately 5 days. To date, the system has been operated at three separate influent concentrations and at various steady temperatures ranging from 4° C to 21° C. Samples drawn from influent and effluent once per residence time have been analyzed for the parameters COD, NH₄, PO₄, and SO₄. Concentration data are presented as a time series moving average for each water quality parameter. All treatments have displayed 80 to 90% COD removal efficiency at temperatures greater than 12° C. Emergent plants have had significant effects upon ammonia and phosphate removal efficiency. Only planted wetland cells exhibited significant long-term phosphate removal. Substantial sulfate removal was expressed in all treatment types at the higher wastewater strengths. Sulfate treatment efficiency was greatly reduced at the lower strength for planted treatments, attributed to increased oxygenation of the root zone. Due to significant water loss through evapo-transpiration, planted treatments generally display greatly improved removal efficiencies if measured on a mass rather than concentration basis.