Uptake of N-15 by macrophytes in subsurface-flow wetlands treating domestic wastewater

Constructed subsurface-flow wetlands are becoming more common for on-site t reatment of domestic wastewater to provide secondary quality effluent. Macr ophytes are generally added to wetlands to increase treatment efficiency bu t their role in reducing N content is controversial. Our investigation util ized two subsurface-flow wetlands to determine the efficiency of different macrophytes in uptake of N-15 labeled ammonium sulfate. Macrophytes in Wetl and 1 recovered 35 % of the added N in their shoots but only 5 % of the add ed N was recovered in the shoots and roots in Wetland 2. A major difference for the two wetlands was N and hydraulic loadings. Wetland 1 received 7.5 Kg N ha(-1)d(-1) and Wetland 2 received 16.9 Kg N ha(-1)d(-1). Retention ti me for Wetland I based on pore volumes was 2.9 d and for Wetland 2 it was 1 .2 d. The retardation factor for NH4+ was approximately 2.5 for both wetlan ds and the breakthrough curves indicated lack of plug flow. The importance of macrophytes in taking up NH4 appeared to dependent on N and hydraulic lo adings.