The cost of government services to citizens during Japan’s long-term economic depression poses a challenge for local governments. To ease the burden of running costs of sewage treatment at the local government level, a waste sludge reduction trial was performed in a real-scale plant by postponing sludge withdrawal from the plant. Sludge reduction was also observed in a small-scale sludge-reduction process. Approximately 30,000 USD of the sludge disposal cost was saved by postponing sludge withdrawal from the plant. Sludge reduction reached approximately 35 %. In terms of CO₂ emissions related to SDGs (Sustainable Development Goals), an 8,000 kg reduction in emissions was observed owing to reduced waste sludge incineration in the sludge reduction trial described in this study.

 Diatoms, unlike green algae and cyanobacteria, are algae that produce 20：5ω3 （EPA）, which is treated as an essential fatty acid. Since EPA is important for the survival and growth of animals, diatoms play an important role as food for zooplankton and aquatic animals in aquatic ecosystems. On the other hand, in recent years, the silica deficiency hypothesis has been proposed, and it has been reported that the outflow of silicon in rivers is decreasing. However, there are no studies evaluating the effect of decreased silicon concentration on diatom growth and EPA production at both population and individual levels.

 Therefore, in this study, we conducted batch culture using the freshwater diatom Nitzschia palea with four levels of silicon concentration （5 to 30 mg/L） and investigated the effects of silicon concentration in the water environment on diatom growth and EPA production. As a result, there was no significant difference in the specific growth rate during the logarithmic growth phase, but a positive correlation was confirmed between population density and biomass （dry weight） production during the stationary phase and silicon concentration. In addition, the dry weight and shell length per cell were significantly different from those of the other systems at the lowest silicon concentration of 4.93 mg/L. EPA contents of population was significantly higher at a silicon concen tration of 29.6 mg/L than at a silicon concentration of 4.93 mg/L. EPA contents per cell was significantly higher in the silicon concentration of 4.93 mg/L than in the other systems. Therefore, it was suggested that diatoms with long shells tended to be distributed at low density in an environment with low silicon concentration, which deteriorated the food environment for consumers.

 It has been reported that diverse macroinvertebrates play an important role on litter decomposition in the vegetated area of a river. To obtain basic biological information on litter decomposition in the vegetated area of a river flowing into Lake Inawashiro, this study investigated the composition of the soil fauna involved in litter decomposition in neutral and acidic rivers. A survey of the soil mesofauna was conducted using the litter bag method. Litter bags with two mesh sizes （0.2 mm and 2 mm） were placed in riparian areas covered with reeds of two rivers （the neutral Nagase River and the acidic Sukawa River） flowing into Lake Inawashiro. The litter bags were removed after 3–12 months. Litter decomposition was in progress because, consistent with previous reports, the total carbon content and carbon/nitrogen ratio in the litter decreased, and the reed residues in the litter bags were softened and disintegrated into pieces, at 12 months after placement. In the acidic riparian area （Sukawa River）, the number of collected taxonomic groups was higher in the litter bag than in the surrounding soil. In the litter bag and the soil, enchytraeids and oribatids were abundant. The collected enchytraeids were assignable to five genera. To understand the role of the soil fauna in litter decomposition, detailed investigation of the physiochemical properties and more comprehensive sampling are required.