Use of a spacer to mitigate concentration polarization during forward osmosis process

• Investigation of the effect of spacer location on concentration polarization (CP)
• Use of spacers to mitigate dilutive internal CP without energy input
• Only spacers with a certain pore size could mitigate CP.

A major barrier for forward osmosis (FO) process is concentration polarization (CP) which dramatically reduces the performance of FO. This study focused on providing a simple method of mitigating CP, especially dilutive internal CP (DICP), in the absence of additional energy consumption and investigate the impact of spacer location on CP quantitatively in both FO (AL-FS) and PRO (AL-DS) mode; finally we tried to provide a spacer location that could best decrease the adverse effect of CP on FO performance.The findings of the research led to the conclusion that in FO mode, placing the spacer (1 mm × 1 mm) in the draw channel with one end connecting the membrane could well mitigate DICP. Besides, placing the same spacer in both feed and draw channels could be an acceptable method of decreasing concentrative external CP (CECP) and DICP simultaneously. If locating the spacer thus, the average water flux could increase by 7.57%, 14.1% and 18.7% when the concentration of the draw solution was 1 M, 2 M, and 4 M separately. Similarly, in PRO mode we found that spacer can also mitigate concentrative ICP and dilutive ECP. Besides, the pore size in the spacer should also be considered when it was used to mitigate CP.