POLARIZATION PHENOMENA IN INTEGRATED REVERSE-OSMOSIS AND MEMBRANE DISTILLATION FOR SEAWATER DESALINATION AND WASTE-WATER TREATMENT

Desalination of seawater and waste water might represent the main impo rtant source of potable water for arid and semi arid zones and for ind ustrialized zones, respectively. Integrated membrane processes permit to produce high quality water by means of desalination of brackish and seawater, and by cleaning and recycling of waste water. The reverse o smosis (RO) process in particular is a well. established and worldwide spread technology. Salt and contaminants can be removed completely an d the process might be less expensive than other separation processes. In RO driving force is represented by a difference between an applied transmembrane pressure (TMP) and the osmotic pressure difference acro ss. the membranes. Highly concentrated solutions cannot be treated as a consequence of a physical Limit imposed by their osmotic pressure va lue. Integration of RO and membrane distillation (MD) permits to overc ome this limit. MD has the advantage not to suffer strong Limitations when high osmotic pressure is involved being the driving force a vapor pressure difference between the two solution membrane interfaces due to the existing temperature gradient. In particular, MD has previously been assessed as being a technically viable process, economically fea sible and competitive with other membrane processes in situations wher e some source of waste energy is available or where electricity is exp ensive. Both process performances might be limited by polarization phe nomena as concentration polarization in RO and temperature polarizatio n in MD. In this latter, only at very high concentration some effects of concentration polarization are present. A complete analysis of pola rization phenomena in RO and MD has been carried out. Film theory and osmotic pressure model theory have been coupled to describe concentrat ion polarization phenomena in RO, taking into account real osmotic pre ssure difference across the membrane due to the accumulation of reject ed solute at the membrane wall. Film theory and Knudsen diffusion in a microporous hydrophobic membrane have been used to describe simultane ous heat and mass transfer phenomena in MD, estimating at the same tim e both concentration and temperature polarization. Theoretical mathema tical models based on thermodynamics and mass transfer phenomena have been elaborated for both RO and MD. The models have been solved numeri cally. Results of the analysis show the physical concentration limits that might be reached with different rejection RO membrane at differen t operative conditions, as applied TMP and fluidynamic regime. These L imits are related especially to solution properties and cannot be over come improving operative conditions. In MD only temperature polarizati on becomes significant and might be responsible for a decay of about 5 0% of ideal fluxes. Only near saturation, concentration polarization b ecomes significant with an influence of about 5% on flux decay. Also i n MD, the analysis has been carried out testing different operative co nditions and fluidynamic regime in feed and permeate side. MD performa nces might be significantly improved by an optimal choice of these con ditions. The integration of the two processes has been studied and som e integrated schemes will be presented.