Pond drying is a traditional method for purifying water bodies; however, its effects have not been extensively studied or quantified. The inhibitory effect and mechanism of pond drying on nutrients released from sediment remain unclear. In this study, the effect of the drying period on bottom sediment and water quality was investigated experimentally and the effect of drying on nutrient released from the bottom sediment was discussed. Sediments were collected from a eutrophic pond, Lake Nanko, Fukushima Prefecture, Japan, for experimentation. The results were as follows. (1) The rate of nutrient release from bottom sediment decreased with a longer pond drying period. After 30 days of drying, the release flux reduction rate was 71% for dissolved-total-nitrogen (DT-N) and 83% for dissolved-total-phosphorus (DT-P) under anaerobic conditions. (2) The decrease in bottom sediment porosity owing to drying reduced the rate of nitrogen release from bottom sediment. (3) Drying reduced the porosity and phosphorus concentration of pore water in bottom sediment, inhibiting phosphorus release from the bottom sediment.

To evaluate the effectiveness of soft CaO-improved soil as tidal flat and shallow sediment material, we conducted a comparative experiment using mesocosm tanks. The soft CaO-improved soil is composed of dredged soil and converter steelmaking slag, which is a byproduct of steelmaking. It is a mixed soil that has been improved by adjusting the particle size and the percentage of the converter steelmaking slag to enable benthic organisms to live in it. In this study, a two-year long-term evaluation using a tidal flat/shallow integrated mesocosm was conducted before application to actual coastal areas. In each experimental tank, soft CaO-improved soil was laid as an experimental site, and dredged soil was laid as the control site. The macrobenthos was observed in the sediments of both the experimental and control sites. More eelgrasses grew in the experimental site, where the light environment was improved as a result of stabilizing the sediment, than in the control site. These results show the utility of the soft CaO-improved soil as the sediment for eelgrass beds and benthic organisms.

In this study, the sewage treatment performance of a multistage constructed wetland introducing a hybrid filtration bed combining tidal flow and partial saturation at the second and third treatment stages was elucidated. The enhancement of the aerobic treatment by the tidal flow was significant, resulting in the attainment of a removal efficiency of more than 97% for BOD and NH₄⁺-N irrespective of the filtration media material. On the other hand, the attainable removal efficiencies for T-N and T-P were 31 and 46%, respectively, when the mixture of zeolite and calcium silicate was used for the second filtration bed media. The results of this study confirmed that a higher sewage treatment performance can be attained for BOD, NH₄⁺-N and T-P removal by introducing the proposed hybrid filtration bed combining tidal flow and partial saturation in the multistage constructed wetland compared with a conventional hybrid system consisting of vertical and horizontal constructed wetlands, although the T-N removal efficiency still remains to be improved.

Even though total nitrogen (TN) and total phosphorus concentrations in semi-enclosed coastal seas have decreased over time, chemical oxygen demand (COD) has not decreased, and conversely, COD increase has been observed. In our previous reports, we showed a stoichiometric change in organic matter (increase in C:N ratio) owing to a decrease in TN in the sea area, suggesting a recalcitrancy of organic matter (hardly decomposed to inorganic matter) . In this report, a similar analysis was conducted in the Seto Inland Sea, and the COD increase due to the recalcitrancy of organic matter was also investigated. The following results were obtained from these studies. TN reduction causes a stoichiometric change and a recalcitrancy of organic matter. Both changes act in a way that does not lower COD. The stoichiometric change in organic matter causes a significant decrease in total organic nitrogen (TON) in the sea, but not in COD. Because of the recalcitrancy of COD, the concentration increases even if the amount of production does not increase. After 1990, the reduction of TN did not reduce the COD in any of the seas investigated, but in contrast, COD increased in highly enclosed waters.

In many developing countries with high temperatures and humidity are suffering from a shortage of safe drinking water. In such countries, dehumidifiers are often used to prevent the growth of rust and mold. The water generated from the dehumidifier (DHW) is often discarded because most people feel that this water is dirty. We continuously investigated the water quality and quantity in Okinawa, which is a hot and humid area, for ～1.5 years. As a result, we found that the water from a dehumidifier (average 11.730 mg L^-1, Main component: TOC 6.985 mg L^-1, NH₄⁺ 3.760 mg L^-1) contained less impurities than tap water (119.821 mg L^-1, Main component: Cl^- 34.315 mg L^-1, Ca²⁺ 28.636 mg L^-1, Na⁺ 25.658 mg L^-1, SO₄^2- 23.293 mg L^-1) throughout the whole year. A compressor-type dehumidifier can generate water of about 62% of rated dehumidification capacity on average annually (6.22 L d^-1) in a closed room, which is superior to a zeolite-type dehumidifier (3.16 L day^-1, about 39% of rating on average annually) . These results show that water generated by dehumidifiers can be used for many purposes such as drinking water with a proper water purification treatment.