Quantitative evaluation of concentration polarization under different operating conditions for forward osmosis process

• Both the ICP and the ECP were quantitatively evaluated under different operating conditions for the first time.
• The ICP always occupied a domain role for the origination of the osmotic pressure drop across the FO membrane.
• Typical ways were adopted and discussed to relieve the adverse effects of the CPs.

Concentration polarization (CP) in forward osmosis is one of the main factors that lower the operating performance of the process. This study quantitatively analyzes the influences of different CPs on the osmotic pressure drop across the membrane under different operating conditions. Results of the study showed as the draw solution concentration increased from 0.25 M to 1.5 M, the osmotic pressure drop proportions caused by the internal CP and the external CP rose from 32.63% to 50.88% and 11.86% to 30.13% respectively, indicating that the influence of the internal CP is dominant and that of the external CP can't be ignored as before especially for high salinity solutions. For relieving the adverse effect of the CPs, the unilateral strengthen of flow rate in draw solution is more meaningful than that in feed solution flow rate, or bilateral strengthen for both solutions. Moreover, the optimal configuration of spacers should be a spacer in contact with active layer in feed channel and a spacer 2.7 mm away from support layer in draw channel. The obtained conclusions in this paper would lay a good foundation for quantitatively understanding of the effects for different CPs and provide a guide for the optimization of the forward osmosis system.