Adverse Impact of Feed Channel Spacers on the Performance of Pressure Retarded Osmosis

Abstract

This article analyzes the influence of feed channel spacers on the performance of pressure retarded osmosis (PRO). Unlike forward osmosis (FO), an important feature of PRO is the application of hydraulic pressure on the high salinity (draw solution) side to retard the permeating flow for energy conversion. We report the first observation of membrane deformation under the action of the high hydraulic pressure on the feed channel spacer and the resulting impact on membrane performance. Because of this observation, reverse osmosis and FO tests that are commonly used for measuring membrane transport properties (water and salt permeability coefficients, A and B, respectively) and the structural parameter (S) can no longer be considered appropriate for use in PRO analysis. To accurately predict the water flux as a function of applied hydraulic pressure difference and the resulting power density in PRO, we introduced a new experimental protocol that accounts for membrane deformation in a spacer-filled channel to determine the membrane properties (A, B, and S). PRO performance model predictions based on these determined A, B, and S values closely matched experimental data over a range of draw solution concentrations (0.5 to 2 M NaCl). We also showed that at high pressures feed spacers block the permeation of water through the membrane area in contact with the spacer, a phenomenon that we term the shadow effect, thereby reducing overall water flux. The implications of the results for power generation by PRO are evaluated and discussed.

Enhanced energy generation and membrane performance by two-stage pressure retarded osmosis (PRO)

“…Therefore, more investigations are needed to focus on the possible multi-stage configurations of the PRO plant. Moreover, many of the prior studies on PRO membrane have employed a crossflow membrane test cell loaded with a small piece of flat sheet coupon [26,[38][39][40]. In those investigations, with such a small scale membrane, the dilution of the draw solution in the crossflow cell was considered negligible, though it could be significant as considerably large area of membrane is required in a PRO process in practice [41].…”

Pressure Dependency of the Membrane Structure Parameter and Implications in Pressure Retarded Osmosis (PRO)

“…The objective of this paper is to present a hypothesis for the interaction between the membrane and spacer which partly builds on the hypothesis of Kim and Elimelech [18]. Based on characterisation experiments we have demonstrated good correlation between measured and modelled membrane performances by applying a pressure dependent structure parameter.…”

“…The calculation of pressure dependent structure parameters in this paper were based on the assumption that the water and salt permeability were independent of the applied pressure, which may appear to be somewhat contradictory to part of the literature [18,19]. Nevertheless, our assumption is based on several arguments.…”

“…Kim and Elimelech [18] have related the deviation between observed and modelled performance to adverse effects between the membrane support and the feed channel spacer. Both membrane deformation and obstruction of water permeation were proposed mechanisms to explain the reduced membrane performance at increasing pressures.…”

When Bioelectrochemical Systems Meet Forward Osmosis: Accomplishing Wastewater Treatment and Reuse through Synergy

“…Because of inherent problems, FO membrane cannot prevent all dissolved draw solute moving from draw solution into feed solution [34], which is called reverse solute leakage [35], another critical shortcoming of FO processes that may decrease the flux and exert negative effects to the post-treatment of wastewater. PRO is a modified FO process with a hydraulic pressure applied in the draw side, and generates the osmotic energy stored in the salinity gradient between saline and fresh waters by releasing the pressurized, diluted salt water through a hydro-turbine [17,[36][37][38]. As estimated by the Norwegian state power company, the amount of osmotic energy may reach 2000 TWh worldwide [36].…”