Performance analysis of plate-and-frame forward osmosis membrane elements and implications for scale-up design

Forward osmosis (FO) studies to date have focused on mass transport and applications. Additionally, further investigations regarding scale-up of FO system are required to commercialize this technology. In this study, the effect of operating parameters on FO performance was experimentally investigated using plate-and-frame FO membrane elements. Operating parameters such as membrane area, concentrations and flow rates of the feed and draw solutions were evaluated to determine their influence on water permeate flux, operating pressures, and water recovery in continuous operation mode. The membrane area was adjusted by series connection of FO elements, to a maximum value of 63 m² (nine elements). The feed flow rate was adjusted to 10, 15, and 20 LPM at draw flow rates of 5 and 10 LPM under various feed (10, 20, and 30 g/L) and draw (70, 110, and 150 g/L) concentration combinations. Increase of operating pressures was observed with increasing feed water flow rates and membrane area. However, the operating pressures of the plate-and-frame elements were significantly lower than that of spiral-wound elements. The lower operating pressure of serial-connected FO elements can be an advantage for a scale-up FO system design. In addition, water recovery data as a function of feed flow rate fraction were compared with model results based on the equilibrium between the feed and draw solutions. Estimation of the normalized membrane area (membrane area to initial feed flow rate) was evaluated to be a critical factor for achieving the desired water recovery. A normalized membrane area higher than 0.08 m² L⁻¹ h is required for a scale-up design of an FO system to obtain the water recovery predicted by the equilibrium model.