Study On Transport And Diffusion Of Concentrated And Heated Brine Discharge

Seawater desalination is becoming a supplement way due to the lack of water resource, but it also lead to adverse environmental effects because of its concentrated and heated brine discharge. The concentrations of the brines are usually found to be double or close to double that of natural seawater. Therefore, it becomes one of the most important environmental problems and much attention was given to the coastal water near the power plant or seawater desalination plant in recent years. By now, few have been reported on researches of the simultaneous simulation and prediction of temperature and salinity fields in complete receiving water with a concentrated and heated brine discharge in place.In this dissertation, a three-dimensional transport and diffusion model is established in order to investigate concentrated and heated brine discharge into coastal water areas so as to predict its influence on ocean environment. The model can be applied to appropriately evaluate the locations of intakes and outlets of the power plants with seawater desalination processes. It is useful for industrial applications of engineering designs and operations of seawater desalination processes to minimize the influence on both coastal water environment and power plant operation efficiency due to heated and concentrated brine discharge.The main conclusions in the dissertation are as follows.1. Based on COHERENS (a three-dimensional hydrodynamic multi-purpose model for coastal and shelf seas) and SWAN (a numerical wave model for obtaining realistic estimates of wave parameters in coastal areas, lakes and estuaries), a three-dimensional transport and diffusion model is developed to simulate concentrated and heated brine discharge into coastal water areas by coupling of COHERENS and SWAN. Wave effect on vertical eddy viscosity coefficient and eddy diffusion coefficient, wave dependent surface drag coefficient and radiation stress are introduced into the coupled model.2. According to the bathymetry of the coastal areas in Jiaozhou Bay and the requirements of computer precision of the interest areas, nested grid is applied to simulate the characteristics of hydrodynamics of Jiaozhou Bay with the coarse, medium and fine resolutions. The simulated time series results by the numerical model have a good agreement with the observed sea surface elevation, depth-averaged velocity and flow directions at selected stations. This indicated that the numerical model provided a fair simulation of the coastal flow in Jiaozhou Bay and were thus available for predicting concentrated and heated brine dispersion in the coastal water areas.3. With particular emphasis on the engineer plant situated on the western coast of Jiaozhou Bay, the distributions of temperature and salinity under pure tidal currents and the combined action of wave and currents are obtained after the concentrated and heated brine discharged from the outlet. It can be seen from the modeled water temperature and salinity elevations, the temperature and salinity rise mainly occurred near the outlet and the temperature and salinity elevations gradually increases from the surface to the bottom. It can be also seen that the temperature contour is similar to the salinity contour, which show that the differences of molecular diffusion between temperature and salinity have little effect on mean flow fields. The distributions of temperature and salinity simulated by the numerical model indicate that transport and diffusion of the concentrated and heated brine mainly are subjected to the actions of tidal flow. Meanwhile, the transport and diffusion is influenced significantly by wave action at nearshore areas with mild slope bathymetry.4. Besides the bathymetry and the hydrological characteristics of the sea, which have significant effects on the range of the transport and dispersion of the concentrated and heated brine, the physical characteristics of the effluent also influence greatly. The nonlinear coupling relationships between temperature and salinity are analyzed by the separate effect of them to the density of seawater, which is calculated by the International Equation of State for seawater. It is concluded that the density increment caused by one unit increase of salinity is larger than the density decrement caused by one unit increase of temperature. Thus, it is necessary to taken into account the nonlinear coupling relationships between temperature and salinity in the case of concentrated and heated brine discharge.5. Three kinds of effluents from the outlet with different combination of temperature and salinity are simulated in order to investigate the nonlinear coupling relationships between temperature and salinity. The simulated results indicated that the effluent with salinity elevation and without temperature elevation would have more adverse effect on ambient environment than concentrated and heated brine. Therefore, it is of more benefit to transport and diffusion of concentrated and heated brine when some temperature elevation is existed between the effluent and the receiving water under the premise that temperature elevation of the receiving water is standard-attainment discharge.