Removal of nitrogen and phosphorus using dominant riparian plants in a hydroponic culture system

The objective of the study was to evaluate potential nutrient removal capacities from eutrophic stream waters using two riparian plants, Phragmites japonica and Salix gracilistyla. The removal efficiencies, removal rates of nutrients (N, P), and their specific growth rate were estimated as a function of inflow loading rate and hydraulic retention time (HRT) in a hydroponic culture system. Weight specific growth rates (WSGR) of P. japonica increased by 0.015 d(-1) in the ambient concentrations of NH4-N, NO3-N, and PO4-P and increased up to 9 times higher than the control (i.e., base concentration), whereas the WSGR decreased at the higher levels of nutrients. Under the same nutrient concentrations, the WSGR in the S. gracilistyla increased up to 9 times higher than the control, and then increased slowly. Nutrient removal efficiencies of both species were higher under lower N and P concentrations and longer HRT conditions. The removal rate of P. japonica had a positive functional relation with surface loading rates of nutrients and leveled off at 3.0 g d(-1) of NH4-N, 12.5 g d(-1) of NO3-N, and 3.0 g d(-1) of PO4-P, respectively. The removal rate of S. gracilistyla also enhanced at the surface loading of 5 g d(-1) of NH4-N, 23 g d(-1) of NO3-N, and 5 g d(-1) of PO4-P, respectively, but was lower than that of P. japonica. It is concluded that the nutrient removal approach using the riparian plants may be used for efficient water quality management in the eutrophic streams with long HRT.