Concentration Polarization Phenomena in Forward Osmosis Process: An Experimental Study

Forward osmosis (FO), is membrane separation technology in its infancy with various applications such as desalination, power generation, in food industry and others. The technology has shown growing interest due to its numerous advantages and its potential for energy-saving. FO process is, however, still facing important limitations related to efficient membrane and draw agent to achieve higher performances. Water diffusion in FO is driven by the osmotic pressure gradient from a highly concentrated stream (draw solution) to a lower concentration solution (the feed stream) across a semi-permeable membrane. Concentration polarization (CP) phenomena have been reported to be the most important factor causing water flux decline in FO process. Reverse solute flux (RSF) is also considered as a major issue in FO. In this work, an experimental study has been carried out for the evaluation of CP phenomena and RSF in FO process. Ammonium bicarbonate and sodium chloride have been used respectively as draw and feed solutions. Results have shown that dilutive internal concentration polarization phenomena (ICP) induced a draw solution osmotic pressure reduction of 42%. Concentrative external concentration polarization (ECP) effects have shown to affect driving force to a lesser extent, the combined effect of ICP and ECP caused a water decline of 51.5%. A concentration profile based on the obtained results across the FO membrane has been defined.