Abstract
Three series of laboratory-scale subsurface vertical flow (SSF) wetland systems with and without forage rice (Oryza sativa L.) were designed for evaluating nitrogen removal performance and biomass production. In these systems, artificial nitrogen-contaminated river water and livestock wastewater were supplied at three hydraulic loading rates (HLRs) using two flow types: flooded continuously SSF and non-flooded intermittently SSF. The average removal efficiencies of total nitrogen (T-N) in flooded SSF systems with forage rice varied from 45% to 77%, whereas those of non-flooded SSF systems were lower, ranging from 18% to 44% in river water treatments. On the other hand, the average removal efficiencies of T-N varied from 27% to 36% in livestock wastewater treatments. A variance analysis showed a significant difference in T-N removal efficiency between systems with and without forage rice. The above-ground biomasses of forage rice in both river and livestock water treatment systems ranged from 1.1 to 4.6 kg dry weight m-2, whereas the amount of nitrogen assimilated by this forage rice ranged from 12.0 to 69.8 gN m-2. Since forage rice containing nitrogen could be supplied to the livestock industry through whole-crop silage, the results indicate that wetland systems using forage rice are useful for the reduction of nitrogen loading in the environment as well as for increasing feed self-sufficiency rate in Japan.
