In the Seto Inland Sea, the largest enclosed sea area in Japan, a gradual increase in the chemical oxygen demand concentrations in the seawater (hereinafter referred to as COD) is apparent, and the achievement ratio of the environmental standards for the COD has not reached 80%. In order to solve this problem, it is necessary to analyze the influence of the open ocean water on enclosed sea areas, in addition to the inflow load and amount of marine-derived organic matter. The present mean values of COD at 0 to 50 and ≥ 100 m depth from the sea surface in open ocean water at the offing of Tokushima (site E9) were 1.2 and under 0.9 mg L^-1. The trend that COD at this site is high value near the surface and decreases with increasing water depth was observed. The COD at site E9 was higher than those from 20 to 30 years ago in the open ocean near site E9; thus, the trend that the COD in the open ocean increased over the past 40 years to the present was confirmed in this study. The trend that the annual mean concentration of COD increased in the midwestern Seto Inland Sea area after the 1980s was obtained from the existent data. In these areas, high positive correlations between annual mean values of COD in some areas (bay and sea) of the Seto Inland Sea and those of background (seawater at the offing of Tokushima and Kochi) were confirmed. These results suggested that the COD in the Seto Inland Sea was affected by the background water, namely, the water of the Pacific Ocean.

View full abstract

The model for exposure assessment of chemical substances, “AIST-SHANEL” ver.3.0, has been available released since October 2015. In this version, river basin data was updated from 2005 to 2011 and a new model for estimating chemical concentrations of any river basin in Japan was added. The model includes 109 class A river systems as well as class B river systems, as well as flow routings of any river basin based on a 250 m grid resolution. The model enables the assessment of the upper area of a river basin at an assessment point designated by the user and yields estimates of the daily concentrations in the area. The calculation results can be displayed on Google Earth and ArcGIS Desktop maps. Unsteady analysis of the model will also be useful for the assessment of the impacts of water quality accidents involving the leakage of chemical substances into river systems. In this study, we showed that the model can be applied to exposure assessments not only under normal conditions but also those of accidents. In the case of accidents, the resilience assessment yields estimates of the degree of recovery of the river water quality deteriorated by chemical leakage.

View full abstract

Under the concept that a sewage treatment plant should serve as a base for the circulation of materials and energy in a watershed, a case study of the Omono River basin in Akita was conducted. Through the improvement of the energy self-sufficiency ratio of Rinkai Sewage Treatment Plant (STP) with wide-area cooperation, the energy self-sufficiency ratio of 31% of the basic system increased to 35% as a result of night soil acceptance. After public sewerage integration, the energy self-sufficiency ratio remained only 28% which is less than that of the basic system, because of the increase in electric energy consumption. However, the watershed STP energy self-sufficiency ratio of the public sewerage integration improved to 28%, which is 9 points more than the 19% of the basic system. From the viewpoint of phosphorus recovery, the 73 kg of the basic system increased to 109 kg after combined treatment with night soil. Furthermore, it became maximum with a phosphorus recovery ratio 25%, which is equivalent to 123 kg, after public sewerage integration. On the basis of the estimates for a model watershed, I suggested that an evaluation of the whole watershed is important, rather than that of an independent sewage treatment plant.

View full abstract

In this study, the impact of rural sewage effluent on the water quality of an agricultural water channel and rural river was investigated. Treated water samples from rural sewage, an agricultural water channel, and a rural river were analyzed. In addition, microflora analyses were conducted on the sediments and water samples from each location using the quinone profile methodology. The results revealed the critical need to study the impacts of treated sewage effluents on freshwater ecosystems. The nitrogen and phosphorus concentrations of water in the agricultural water channel increased downstream of the treated wastewater (TW) inflow. The annual average nitrogen and phosphorus concentrations in the TW were 7.33 mg L^-1 and 1.83 mg L^-1 higher, respectively, than those measured in the agricultural water channel upstream of the reclaimed wastewater inflow. Furthermore, a slight increase in the sediment microbial biomass, determined by the loss-on-ignition method, was observed in the treated water flowing into the agricultural water channel. It was clarified that rural sewage effluents with a high concentration of nutrients affect the microflora in the agricultural water channel, and, in particular, contribute to the increase in the amount of denitrifying bacteria.

View full abstract