Direct nanofiltration treatment using flat-sheet membrane modules reconfigured from spiral-wound elements

Direct nanofiltration (NF) treatment without pretreatment has attracted attention for achieving low-energy advanced drinking water treatment in drinking water applications. However, the validity of submerged NF systems for large-scale modules and variable feed water quality remains unexplored. This study aimed to assess the viability of pump-operated and gravity-driven membrane (GDM) filtration of lake water without pretreatment using NF membrane modules reconfigured from commercially available NF membrane elements typically suited for pressurized operation. The permeate flux of the pump-operated NF system gradually declined from 2.3 to 1.2 L/m 2 h at a feed temperature of 7–11°C until 12 d; thereafter, the permeate flux remained stable until 36 d. Cleaning the membrane surface by sponge wiping achieved near-full recovery of the permeance. The removal of dissolved organic matter and conductivity were relatively low, at 30–50 % and 10–30 % respectively. The specific energy consumption (SEC) of the pump-operated NF system could be as low as 116 Wh/m 3 , based on the pilot-scale tests. In contrast, the permeate flux of GDM filtration using the reconfigured NF membrane modules remained constant at approximately 0.28–0.37 L/m 2 h at a feed temperature of 5–22°C for over 78 d despite the limited water head available. The separation performance of GDM filtration was comparable to that of the pump-operated NF system, while the removal of conductivity (ions) was negligible. Despite the limitations of permeance and separation performance, both pump-operated and gravity-driven systems with reconfigured NF membrane modules have the potential to achieve low-energy and decentralized water supply systems. • 4-in. spiral-wound nanofiltration (NF) membrane elements were modified. • A submerged filtration system comprising ten modified NF modules was assessed. • Permeate flux during pump operation decreased until day 12, and then it stabilized. • Specific energy consumption of the pump-operated NF system could be 116 Wh/m3. • The gravity-driven submerged NF system did not exhibit flux decline over 78 days.