This project is focused on the study of concentration of desalination brines, specifically those generated on the desalination of brackish waters using reverse osmosis. When concentrating brines two different objectives are pursued. The first one is to concentrate the brine up to reach the maximum salts concentration, close to saturation, to facilitate the subsequent recovery by a crystallization step. This option is recognized as Zero Liquid Discharge as all the water is evaporated and the solids dissolved are recovered. The second possible objective is the so-called near-ZLD (near Zero Liquid Discharge), consisting in reducing considerably the volume of brine achieving a partial evaporation of the water. The second one is the objective aimed in this project.
As concentration step Membrane Distillation (abbreviated hereafter as MD), an emerging technology integrating membrane and thermal technology will be evaluated. Specifically, the technology to be used is Direct Contact Membrane Distillation (abbreviated as DCMD), in which a hot feed concentrated solution is put in contact with one side of a hydrophobic membrane, on the other side a cold pure water stream is circulated.
The main two advantages of MD are the possibility to operate at low temperatures (e.g. 40 to 60oC) and the capability of working with high concentrations in the feed solution. From the possible problems facing MD three of them have been studied: scaling, temperature polarization and concentration polarization. Scaling consists in the deposition of solid salts on the membrane layer; blocking it and thus decreasing the flow of permeate. In order to avoid the early precipitation of salts different pre-treatments of the brine have been studied and experimented with. To study polarization effects, experiments of distillation modifying operational parameters have been carried out.
The experimental set-up consisted in a membrane module connected in the hot-side to a heated bath, where the feed solution is submerged, and on the cold- side to a refrigerated bath where the distilled solution is cooled down. The mass of distilled water and the temperature in the inlets and outlets of the module have been continuously measured.
To study the influence of the brine pre-treatments an aeration column was built. Brine was treated with different combinations of aeration and acidification. Concentration of the feed solution along the distillation experiments has been analyzed to identify the optimum brine pretreatment.
Experimental data collected in the different experiments have been evaluated according existing theoretical models, being able to evaluate the DCMD method effectiveness to concentrate RO brine, determinate the concentration factors achievable for each pre-treatment, study polarization effects and estimate membrane transport parameters, as well as efficiency indicators.
