Pressure retarded osmosis for osmotic power production: influence of internal concentration polarization and membrane fouling

• Main Author: She, Qianhong
• Other Authors: Tang Chuyang
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Environmental engineering::Water treatment
• Online Access: http://hdl.handle.net/10356/55431
• Institution: Nanyang Technological University

Pressure retarded osmosis (PRO) has attracted increasing interest in recent years due to its potential for harvesting the renewable and sustainable osmotic energy from the mixing of two water streams with different salinities. However, the performance of PRO process is strongly affected by the internal concentration polarization (ICP) and membrane fouling. This study characterized ICP and fouling in PRO processes by varying the operating conditions (such as draw and feed solution concentration and composition, membrane types, membrane orientation, temperature, and feed spacer geometry). The phenomenon of reverse solute diffusion from draw solution to feed solution and its adverse effect on ICP, fouling, and PRO performance were also investigated. The commercial cellulose triacetate (CTA) forward osmosis (FO) membranes were used in all the PRO experiments and the alginate was selected as a model foulant to study membrane fouling. It was found that the PRO performance was improved by decreasing the feed solution concentration, increasing the draw solution concentration, orientating the membrane with its active layer towards the draw solution (AL-DS), and increasing temperature. The membrane with higher water permeability, lower solute permeability and lower structure parameter performed better in PRO process. It was also observed that membrane under PRO operation underwent severe deformation. Theoretically, the extent of membrane deformation is proportional to the applied pressure and the square of the feed spacer opening size and inversely proportional to the membrane mechanical strength. The severe membrane deformation led to the deterioration of the membrane separation properties and the drastic increase of the reverse solute diffusion, which results in enhanced ICP and thus the substantially reduced PRO performance. Although the AL-DS membrane orientation exhibited greater PRO performance compared to the alternative orientation, it was found that this orientation was more prone to fouling. It was also found that reverse solute diffusion can enhance the membrane fouling. When the draw solution (DS) contained large quantities of fouling promoters (i.e., Ca2+ and Mg2+), severe organic fouling was observed. The effect of reverse solute diffusion enhanced fouling is related to (1) the type of draw solute and the rate of its diffusion into the feed solution (FS), and (2) the ability of the diffused draw solute to interact with foulant in the feed solution. It was also found that the increase of DS concentration exacerbated the PRO fouling due to the synergistic effects of increased reverse solute diffusion and increased initial water flux level. However, the increase of applied hydraulic pressure mitigated the alginate fouling for NaCl draw solution but exacerbated the alginate fouling for CaCl2 draw solution due to the competing effects between the increased reverse solute diffusion and the reduced initial water flux. It was further found that the alginate fouling was more severe under PRO operation than that under forward osmosis (FO) operation at an identical initial water flux level using seawater-based DSs due to the faster reverse solute diffusion under PRO operation. Severe membrane fouling can also lead to the enhanced ICP, which causes additional water flux decline.