Relating membrane surface properties and flux recovery during the chemical cleaning of forward osmosis membrane

In this study, we evaluated the efficiencies of chemical cleaning agents for the removal of foulant and related the changes of membrane surface properties to the flux recovery in forward osmosis (FO) membrane process. Commercially available chemical cleaning agents were added and processed according to the protocols of reverse osmosis (RO) membrane processes into two steps—acid treatment, then base treatment, and the flux recovery and membrane surface properties were tested in FO mode. The flux of the fouled membrane was only 23% of initial water flux tested in FO mode, and the addition of cleaning chemicals could improve the water flux to be 65 and 83% of initial water flux after each step, respectively. The reverse flux selectivity of fouled membrane was increased up to 61 and 86% after each cleaning steps due to the decrease in the concentration polarization inside the layer. The removal of biofouling layer during each cleaning step was demonstrated by scanning electron microscopy (SEM) and FT-IR, while incomplete removal of colloidal matters on membrane surfaces was observed. The total membrane surface free energy was increased after the cleaning steps due to the increase in electron-donating and electron-accepting character. Overall, chemical cleaning agents designed for RO could remove various membrane foulants as well as recover the hydrophilicity of membrane surfaces and consequently be applied for the cleaning of membranes operated in FO process, but additional cleaning protocol should be developed for the removal of colloidal matters from membrane surfaces.