Abstract
A water quality tank model based on the water and nitrogen balances in Tone basin of Gunma Prefecture was developed to estimate nitrogen concentration of the river and used to discuss the way to reduce the nitrogen load. The water quality model consists of sixty tanks. The linkage of the tanks was determined by structure of rivers in Gunma Prefecture. The difference between the nitrogen loads generated in the basin and discharged into Tone River was given as diminution by denitrification and / or adsorption. The suppositional height of the tanks and the rate constant of nitrogen diminution were determined by the method of parameter fitting. The values of the tank height and rate constant were 7 .0 m and 1.5×10-4/day, respectively. The simulation was carried out to discuss the way to reduce the nitrogen load using four scenarios obtained from the historical change of the nitrogen load. The calculated results indicated that nitrogen load generated from both rural and urban areas affected the water quality of Tone River. It was considered that recycling of livestock waste could be best way in the rural area of a high nitrogen load. In addition, utilization of livestock waste as energy source and treatment of the waste by advanced nitrogen removal process were proposed . In the other area, improvement of wastewater treatment plant is recommended to reduce the nitrogen load generated from anthropogenic activities.
