Membrane fouling and long-term performance of seawater-driven forward osmosis for enrichment of nutrients in treated municipal wastewater

• High nutrients separation performance was achieved in a long-term operated FO.
• Mild fouling impact was observed in the long-term FO without chemical cleaning.
• Biofouling was identified as primary FO membrane fouling by municipal wastewater.
• FO membrane fouling control mechanism by feed disinfection was explained.

Forward Osmosis (FO) has been designed and studied for various applications due to the advantages it offers when compared to traditional pressure-driven membrane processes. In our previous study, seawater-driven forward osmosis was successfully applied to concentrate nutrients in treated municipal wastewater. In this study, the long-term operational stability and performance of the seawater-driven FO process was investigated by a 2-month continuous operation of bench-top FO reactors. Nitrification inhibition was recommended to maintain the concentration performance of nitrogen by preventing ammonia from being oxidized by microorganisms. With nitrification inhibition, concentration performances of 3- to 4-fold of total phosphorus and 2.1-fold of ammonia were stably achieved at a feed volume reduction of 75%. Membrane fouling, with the active layer of the FO membrane facing the feed solution, had only a mild impact; there was approximately 5% water flux reduction but no significant effect on nutrient enrichment performance. The analytical results by FTIR and SEM-XMA suggested that bio-fouling with a combined structure of bacterial clusters, biopolymers, and slight inorganic scales was the main FO membrane fouling pattern. Furthermore, pre-disinfection performed in feed solution effectively retarded the occurrence of membrane fouling in the FO process.