An experimental study of performance parameters and ion concentration along a reverse osmosis membrane

This paper essentially deals with how different important membrane performance monitoring parameters and ion concentrations change along a reverse osmosis (RO) membrane when used for seawater desalination. In order to study this issue, one long membrane was simulated by a sequence of five membranes in series. In the experiments, the concentrate was collected, and used as a feed for the next stage. It was found that both the recovery and the permeate flux decreased along the membrane as a result of the decrease in the apparent pressure driving force resulted from higher osmotic pressure of the feed. Moreover, this elevated osmotic pressure was the reason for the increase of both the stage and the cumulative permeate concentrations along the membrane. For the ions studied, it was found that the permeate ion concentration increases almost linearly along the membrane for Na+, Ca2+, Mg2+, HCO3− ions and this increase became steeper for other ions, namely, K+, Cl− and SO42− at later stages of the membrane length. However, a nearly 24% was the average increase in all the ion concentrations along the total membrane length, that is 229 cm which corresponds to a nearly 5% increase per stage, 45.8 cm, if a linear increase would be assumed.