Experimental investigation and mathematical modelling of a spiral wound membrane module for osmotically assisted reverse osmosis applications

Osmotically assisted reverse osmosis (OARO) has gained interest for applications like desalination, wastewater treatment, and draw solution recovery in forward osmosis. Despite that, on the one hand limited experimental research on pilot- or industrial-scale modules is available, and on the other hand existing simulations are often theoretical and unsuitable for real OARO scenarios. A validated mathematical model reflecting the actual membrane behaviour is crucial for understanding the process accurately and designing it effectively. This study investigated a prototype spiral wound 4040 module for OARO in a pilot plant, using NaCl solutions at various concentrations as feed and sweep solutions. Experiments were carried out at feed concentrations from 17 g/L to 226 g/L and sweep solution concentrations equal to or less than the feed. Steady-state water flux data were collected applying pressures from 8 to 28 bar. In parallel, a predictive mathematical model based on material balances was developed and validated to describe water flux variation within the membrane module, providing insights that are useful for process optimization. Positive water fluxes (1-2 LMH) were obtained even for highly concentrated solutions (226 g/L), indicating the system's capability to handle high salinity feeds. It has been observed that deformation of the membrane under high pressures can adversely affect its performance in a significant way. This is because deformation reduces the cross-sectional area of the membrane and increases the pressure drop on the sweep side. In conclusion, the results demonstrate promising membrane capability in managing high solute concentrations, and the validity of the developed predictive model.