Fouling mitigation for cellulose triacetate and thin-film composite forward osmosis membranes for seawater desalination by sodium metasilicate sol-gel

Membrane fouling in osmosis processes is less severe than in the pressure-driven processes, but it is inevitable over time. Physical or chemical cleaning methods, or a combination of both, are used for fouling mitigation. This study investigated membrane fouling in the forward osmosis (FO) membrane for seawater desalination by sodium metasilicate (SMS) sol-gel. Cellulose triacetate (CTA) and thin film composite (TFC) FO membranes were tested for fouling and cleaning in 3 cycles of 5 and 24 h. For 5-hour tests with the active membrane layer against the draw solution, TFC and CTA membranes experienced severe fouling due to silica and inorganic fouling despite cleaning with docusate sodium. Membrane fouling was reduced in the TFC membrane tested with the active layer against the feed solution. For 24 h tests, TFC fouling was reduced in the 2nd and 3rd cycles to 4 % and 1 % following docusate sodium cleaning and osmotic backwash at 40 °C. Cleaning with NaOH at pH 11, followed by osmotic backwash at 40 °C, reduced TFC fouling to 4 % and 0.24 % in the 2nd and 3rd cycles caused by silica and iron oxides. However, cleaning with EDTA had the highest flux decline at 4 % and 3 % in the 2nd and 3rd cycles, caused primarily by calcium sulfates and calcium silicate fouling. The results demonstrate the importance of combining physical and chemical cleaning for fouling mitigation in the FO processes and to reduce chemical use.