The objectives of this study are to establish a runoff model and show pollutant loadings discharged into Lake Biwa by simulating the change in pollutant runoff in proportion to the progress of sewer construction. The runoff model, hereinafter referred to as “Macro Model”, consists of three main sections. The first one shows pollutant runoff from point sources taking into consideration the sedimentation of pollutants in sewers. The second and third ones show pollutant runoff from nonpoint sources in an urban and rural area taking land use into consideration.
As a result of the simulation on nutrient runoff, the distribution of pollutant loadings in Lake Biwa basin and their runoff characteristics, which were changing in response to sewer construction in the basin, were shown. And it was cleared that the ratio of nonpoint pollutants to integrated pollutant loadings discharged from all sources into Lake Biwa should be 0.8% for T-N and 3.1% for T-P after sewer system achieve 100% coverage. Therefore, it was proved to be significant to control these pollutants discharged during storm events.
In the result, some available knowledge concerned with the environmental management of water in Lake Biwa was obtained.

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Water quality deterioration in lakes is caused by social and economic factors. For future water environment preservation it is necessary to analyze integratedly the constituents of the catchment area and to structurally remove the cause of water pollution. The method of total management of the catchment area was studied here about Lake Teganuma.
The catchment area of Lake Teganuma was divided into 168 small sections, and multiple relationships among information such as land utilization, population density, extension rate of sewerage and effluent load of chemical oxygen demand, total phosphorus and total nitrogen were examined.
High multiple relationships with correlation coefficients of 0.924-0.997 were obtained among the effluent load density of chemical oxygen demand, total phosphorus and total nitrogen, the population density using onsite small scale nightsoil treatment facilities and those using nightsoil treatment facilities. And high multiple relationships with correlation coefficients of 0.630-0.867 were also obtained among the effluent load density, the total population density and the extension rate of sewerage. These reflected the fact that the major constituent of effluent load to Lake Teganuma is domestic waste water. As the result, sections where water purification measures should he preferentially taken could be grasped.
In order to solve this problem fundamentally, it is necessary in future to manage synthetically the catchment area, that is, to review the present sewerage system planning and the present land utilization planning including city planning.

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The authors investigated the water quality in Lake Barato near downtown Sapporo to the characteristics of eutrophication of lakes in the vicinity of urban areas. We found that (1) the eutrophication of Lake Barato, which has been affected by surrounding waters, could be statistically classified into three groups, (2) the algae concentration of the each group depended on the concentration of phosphorus, especially particulate phosphorus, and (3) among particulate phosphorus, reactive phosphorus may control the growth of algae. Due to seasonal changes and the differences in location of the concentration of phosphorus compounds, the concentration of algae in the lake could potentially increase twice as much as now.

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Degradation characteristics of blue green alga, Microcystis viridis producing toxic substance Microcystin by Mastigophora, Monas guttula were examined using an aquatic large scale microcosm. The viable cells of M. viridis was effectively predated and degraded by the small flagellate within a wide water temperature range from 20°C to 30°C. In the removal effects of several important water parameters in eutrophicated waters, the removals of chlorophyll-a and particle COD originated from M. viridis were 90% and 80%, respectively in a week. Moreover, the toxic substance, Microcystin RR, YR and LR were perfectly degraded within a short time. It was suggested that microanimals such as small flagellate played an important role for the disappearance of water bloom formed by Microcystis in eutrophicated lake.

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The effects of BOD₅/TKN ratio in raw night soil and dissolved oxygen concentration in a U-tube bioreactor on nitrogen removal and N₂O formation were studied using a single-sludge acivated sludge system including a U-tube bioreactor with a membrane filter for the separation of activated sludge from purified night soil. The advantage of this system is that high nitrogen removal efficiency is achieved even at a low BOD₅/TKN ratio in night soil without additional carbon source by the high MLSS (average MLSS of approximately 20 kg·m^-3) operation. Night soil treated by the U-tube bioreactor in a pilot plant had an average BOD₅/TKN ratio of 1.75. It was found out that accumulation of NO₂^- in the U-tube bioreactor, high N₂O formation and inhibition of nitrifying organisms took place at BOD₅/TKN ratio of 2 in night soil by the conventional intermittent aeration mode in which the dissolved oxygen concentration was kept over 2 mg·l^-1 during the aerobic period. However, the reduction of NO_x-N and control of N₂O formation were achieved completely by the low dissolved oxygen concentration (between 0 and 1 mg·l^-1) mode in which the average BOD₅/TKN ratio and TKN load were 1.75 and 0.05 kg·kg^-1·d^-1, respectively.

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To improve water quality of a lake, the domestic wastewater pollutants reduction and non-point source pollutants reduction in those are flowing into the lake is necessary. Because a countermeasure to non-point source pollutants control is not had done enough, we must recognize an influence of non-point source pollutants on water quality of the lake and we must execute an appropriate countermeasure against them. There, at first, we examined an influence of the domestic wastewater and non-point source pollutants on water quality of the river which is flowing in the model basin. At this examination, we applied “Simulation System for Evaluation the Environmental Condition of the Basin”. By means of this simulation system, we can simulate the production of pollutants at point sources and non-point sources in a basin and simulate runoff processes of these pollutants. From these simulation results, we made the following matters clear. A present water quality of the river is greatly influenced by the domestic wastewater. An execution of a countermeasure against the domestic wastewater is necessary for water quality improvement. It will be important that the countermeasure for non-point source pollutants reduction by the execution of the countermeasures against the domestic wastewater pollutants. Moreover, we proposed many control measures of the domestic waste water and non-point source pollutants to improve water quality of the lake.

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Denitrification rates were measured by acetylene blockage technique in slurried material collected from subsurface soil under paddy fields in Okabe, Saitama, under anaerobic, nitrate-amended conditions in the laboratory. The subsurface soil from 40-150cm showed the rates of 0.4-46ng N·g^-1 wet soil·h^-1, which were 1-2 orders magnitude smaller than those of the surface soils. The results demonstrate that denitrification can occur in subsurface soil under paddy fields in anaerobic conditions and can serve as a mechanism for nitrate removal from groundwater there.

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