A modeling and experimental comparative study of a closed-loop forward osmosis-nanofiltration and standalone nanofiltration systems for fouling-prone wastewater treatment

[Display omitted] •A module-scale mathematical model is developed for the FO-NF hybrid system.•Experimental data are obtained for FO-NF hybrid system with divalent draw solutes.•Hydraulic pressure and draw concentration control the NF retentate concentration.•Determined conditions where NF retentate may be directly used as a draw solution.•FO-NF hybrid performance is better than Standalone NF for fouling waters. The forward osmosis-nanofiltration (FO-NF) process presents a potential alternative for the treatment of wastewater effluents, compared to pressure-driven systems like reverse osmosis (RO)/nanofiltration (NF). This work bridges the gap in exploring the impact of continuous draw solute regeneration in FO-NF hybrid, and its influence on the performance over standalone NF at module-scale. The Speigler-Kedem model, along with the concentration polarization model, is used to describe the local mass transport in the membrane. Following this, the combined mathematical models for FO and NF are integrated into the hybrid FO-NF system at module-scale. Subsequent process optimization reveals that NF hydraulic pressure and inlet draw concentration control the NF retentate concentration and then simulate the desired operating conditions for the hybrid system when the NF retentate stream is directly used as a draw solution for the FO process. Further results show that in non-fouling feed solution, the FO-NF system is inferior to standalone NF in terms of SEC. In contrast, for fouling-prone feed solution, the standalone-NF performance degrades with time, which gradually mitigates water recovery from 40 to 15.6 % while increasing the SEC from 0.89 to 2.38 kWh/m3. At the same time, the FO-NF hybrid system performs better with Na2SO4 and MgSO4 as draw solutes with nearly constant water recovery of 32.6 % and 28.6 %, and negligible SEC fluctuations of 3.26 kWh/m3 and 4.70 kWh/m3 for the two draw solutes respectively. Despite this, compared with other state-of-the-art desalination technologies, the FO-NF hybrid exhibits comparably lower water cost (average 0.31 USD/m3) and effectively handles fouling-prone wastewater.