Polarization phenomena in integrated riverse osmosis and membrane distillation for seawater desalination and waste water

Desalination of seawater and waste water might represent the main important source of potable water for arid and semi arid zones and for industrialized 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 osmosis (RO) process in particular is a well established and worldwide spread technology. Salt and contaminants can be removed completely and 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 across the membranes. Highly concentrated solutions cannot be treated as a consequence of a physical limit imposed by their osmotic pressure value. Integration of RO and membrane distillation (MD) permits to overcome 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 feasible and competitive with other membrane processes in situations where some source of waste energy is available or where electricity is expensive. Both process performances might be limited by polarization phenomena as concentration polarization in RO and temperature polarization in MD. In this latter, only at very high concentration some effects of concentration polarization are present. A complete analysis of polarization phenomena in RO and MD has been carried out. Film theory and osmotic pressure model theory have been coupled to describe concentration polarization phenomena in RO, taking into account real osmotic pressure difference across the membrane due to the accumulation of rejected solute at the membrane wall. Film theory and Knudsen diffusion in a microporous hydrophobic membrane have been used to describe simultaneous heat and mass transfer phenomena in MD, estimating at the same time both concentration and temperature polarization. Theoretical mathematical models based on thermodynamics and mass transfer phenomena have been elaborated for both RO and MD. The models have been solved numerically. Results of the analysis show the physical concentration limits that might be reached with different rejection RO membrane at different operative conditions, as applied TMP and fluidynamic regime. These limits are related especially to solution properties and cannot be overcome improving operative conditions. In MD only temperature polarization becomes significant and might be responsible for a decay of about 50% of ideal fluxes. Only near saturation, concentration polarization becomes significant with an influence of about 5% on flux decay. Also in MD, the analysis has been carried out testing different operative conditions and fluidynamic regime in feed and permeate side, MD performances might be significantly improved by an optimal choice of these conditions. The integration of the two processes has been studied and some integrated schemes will be presented.