Diffusion/dilution of desalination discharges into seawater

Abstract

Holographic interferometry (HI) is an optical technique, by means of which changes in the refractive index can be visualized as interference fringes. The technique has been used to study the behavior of brine discharges from desalination plants into the sea. When the brine discharges in the sea, the diffusion/dilution of the salts in the seawater begins thus causing concentration changes associated to changes in the refractive index. This variation of the refractive index can be visualized as an interference fringe pattern by means of HI. In the present work, the first aim has been to determine the diffusion coefficient of brine in seawater. Diffusion experiments, consisting on slowly introducing brine in spectrophotometric cell containing seawater, have been carried out. When both solutions get in contact, the diffusion of solute starts and interference fringes (interferogram) begin to appear. The diffusion coefficient of brine in sea water has been calculated using these interferograms and a value of 1.27•10-5 cm2/s has been obtained. Dynamic experiments have also been carried out, introducing a flow of brine into a transparent container with seawater. Several characteristics of the brine discharge (flow rate, concentration, depth at which is poured and angle of discharge) have been studied. Using the interferometric fringe patterns obtained, concentration profiles in the container have been calculated. The results obtained show a better diffusion/dilution of the brine when the discharge is not horizontal and a moderate flow rate is used.