Single-pass forward osmosis for efficient feed concentration: optimizing multiple modules arrangement and flow distribution

Abstract

Forward osmosis (FO) is gaining prominence as a concentration process. However, most systematic studies remain limited to setups involving recirculating feed solution (FS) and draw solution (DS), and employing small membrane coupons or single module, thereby limiting insights into high-recovery concentration performance at the system scale. Realizing the full potential of FO for concentration purposes and maximizing osmotic energy efficiency requires a detailed understanding of module arrangements, flow dynamics, and their influence on operational scalability. This study presents pilot-scale investigation of FO modular arrangements, including series and tree configurations operated in single-pass mode under both co-current and counter-current flow orientations, combined with MATLAB-based mass transfer modeling. Experiments used Aquaporin HFFO2 hollow fiber (HF) modules and tested both low-osmotic-pressure (DI water) and high-osmotic-pressure (5 g/L saline) FS. The model showed strong agreement with experimental results and enabled detailed evaluation of spatial performance variations, enhancing evaluation of system scalability and efficiency. The full potential of countercurrent flow emerged in multi-module setups. With a 5 g/L FS, series and tree arrangements initially performed similarly, achieving FS concentration factors of 4.59 and 5.10, respectively. Simulations revealed that water permeation in the series arrangement progressively diluted the DS, increasing its volumetric flow and leading to hydraulic imbalances that limit the system's ability to handle long-term stable operation. The tree arrangement faced similar challenges under co-current flow, but counter-current operation mitigated them by ensuring balanced flow distribution across stages, sustaining stable flux (4.76 -6.71 LMH) and achieving an overall recovery of 80.40 %. To further enhance the system performance, the impact of hydraulic conditions was explored through simulations to identify operating regimes with optimal trade-offs