Evaluation of useful plants for the treatment of polluted pond water with low N and P concentrations

Eutrophication of ponds for agricultural use has begun to adversely affect rice production and the residential living environment in Japan. Cultivation of useful terrestrial and aquatic plant species in plant-bed filter ditches enables to treat domestic wastewater in addition to resource recycling and amenity functions. We evaluated the ability of several plant species, Italian ryegrass (Lolium multiflorum Lam.), hanana (Brassica campestris L. var.), African marigold (Tages erecta L.), sorghum (Sorghum vulgare Pers.), kenaf (Hibiscus cannabinus L.), papyrus (Cyperus papyrus L.), and reed (Phragmites communis Trin.), to remove nitrogen and phosphorus from polluted pond water whose N and P concentrations were much lower than those in domestic wastewater. Artificial pond water containing 2.5 mg L^-1 of N and 0.5 mg L^-1 of P was supplied to ditches at a loading rate of 1.1 g m^-2 d^-1 for N and 0.21 g m^-2 d^-1 for P. Italian ryegrass, papyrus, or kenaf in ditches removed N and P more effectively than other plants. The average removal rate of Italian ryegrass in ditches was 0.62 g m^-2 d^-1 for N and 0.10 g m^-2 d^-1 for P, that of papyrus 0.81 g m^-2 d^-1 for N and 0.15 g m^-2 d^-1 for P, and that of kenaf 0.73 g m^-2 d^-1 for N and 0.11 g m^-2 d^-1 for P. The influence of N and P concentrations on the removal rates of Italian ryegrass, papyrus, and kenaf was studied. Concentrations at a removal rate of zero were 0.2–0.3 mg L^-1 for N and 0.01–0.03 mg L^-l for P. At low concentrations, the plant removal rates increased sharply with the rise in the concentration. When N concentrations exceeded 0.3–0.6 mg L^-1 and P concentrations 0.10–0.45 mg L^-1, the removal rates were high and less affected by the concentration. It was considered that these plant species could be used most efficiently at the concentrations where removal rates are less restricted.