Evaluation of electrodialysis for scaling prevention of nanofiltration membranes at high water recoveries

The water recovery of nanofiltration in drinking water production is limited to 80–85%. When the water recovery is increased, there is a risk of scaling of sparingly soluble salts, such as CaSO4 or CaCO3, onto the membrane surface. There is a need for robust technologies that handle the problem of mineral scaling in nanofiltration and reverse osmosis, allowing operation at higher recoveries, i.e., with a higher production of potable water. In this study, the retentate stream of a nanofiltration unit was therefore desalinated by electrodialysis. Two different ion exchange membrane pairs, namely AMX-CMX (Neosepta, Japan) and FTAM-FTCM (Fumasep, Germany) were used for this purpose. The membrane pairs were compared on the basis of their removal efficiency of the main ions present in natural waters, with special attention to calcium and sulphate ions. The economic feasibility of retentate treatment by electrodialysis is discussed as well. The FTAM anion exchange membranes of Fumasep were able to remove sulphate ions faster, relative to chloride or nitrate ions. This is unexpected, because sulphate ions have a high hydrated ionic radius and steric hindrance typically obstructs their transport through anion exchange membranes, as is the case with the AMX membranes. This feature makes the FTAM membranes appropriate for the desalination of retentate streams of nanofiltration and reverse osmosis membranes, in water recycling applications. The other membranes can be regarded as non-selective.