Numerical Study on the Effects of Water Surface Shield and Oxygen Injection on Water Quality in Lake Biwa

Abstract

Numerical simulation was carried out by means of a three-dimensional hydrostatic and ecosystem coupled model in order to predict the effects of water surface shield and oxygen injection on the water quality in Lake Biwa. Recent observations show the degradation of water quality particularly around the water bottom in Lake Biwa. One of the possible reasons of the degradation is the depletion in the vertical mixing of waters due to climate change. The meteorological observations at Hikone show that the atmospheric temperature has increased by 1°C in the recent 20 years. If the vertical mixing of waters is weakened, the waters with rich oxygen are not supplied enough around the water bottom during winter seasons, and the concentration of dissolved oxygen is hardly recovered to the saturated level. The depletion in the vertical mixing of waters has been increasingly worried since Intergovernmental Panel on Climate Change (2007) reported that the globally mean atmospheric temperature will increase by a few °C in a century. In order to cope with such problems, the present study proposes a feasible system to inject the oxygen produced by the electrolysis of water. In the proposed system, water surface is shielded by a floating platform, which blocks off the penetration of solar radiation through the water surface and generates the density-driven current due to the horizontal difference in water temperature. In addition, solar cell modules are installed on the floating platform to produce oxygen by means of the electrolysis of water. The produced oxygen is then released from the electrodes installed around the water bottom. As a result of numerical simulation using the three-dimensional hydrostatic and ecosystem coupled model, the effect of water surface shield was negligible since the surface area of the floating platform was quite smaller than that of Lake Biwa. The effect of oxygen injection was remarkable and localized in summer than in winter. The concentration of dissolved oxygen was then expected to increase by more than 0.2mg/l within 15m from the bottom surface in summer, in the cases when 730,000m3 of oxygen was injected in an year. The effect of seasonal change in the production of oxygen was negligible since little difference was found in the concentration of dissolved oxygen. As future studies, the nested grid system should be utilized to predict precisely the diffusion of oxygen both in the vicinity of the electrode and in the whole Lake Biwa.