Forward osmosis for multi-effect distillation brine treatment: Performance and concentration polarization evaluation

The discharge of reject brine from seawater desalination processes is a threat to marine ecosystems. This study investigated the feasibility of forward osmosis (FO) for the treatment of reject brine from multi-effect distillation (MED) systems. The performances of two commercial FO membranes (ie, cellulose triacetate (CTA) and polyamide thin film composite (TFC) membranes) were compared. The effects of operating conditions, such as draw solution concentration, cross-flow velocity, and temperature, on concentration polarization and consequently on the water flux, were quantitatively analyzed using a mathematical model. Results showed that the batch FO process could effectively reduce the volume of MED brine to 54.9% using 3 mol/L NaCl. As the draw solution concentration increased from 1-5 mol/L, a significant increase in the initial water flux from 3.23-17.88 L center dot m(-2) center dot h(-1) and from 3.55-24.04 L center dot m(-2) center dot h(-1) was observed for the CTA and TFC membranes, respectively. However, the proportions of the effective osmotic pressure differences decreased from 20.7% to 11.9% and from 23.5% to 6.2% for the CTA and TFC membranes, respectively, indicating the concentration polarization (CP) was severe for high-salinity brine treatment. The positive effects of increasing cross-flow velocity on CP were limited. Moreover, the high temperature of the MED brine effectively mitigated the internal concentration polarization (ICP), thereby enhancing the water flux. Overall, this study provides valuable guidance for the application and optimization of FO in MED brine treatment.