Pressure Assisted Osmosis (PAO) to enhance Forward
Osmosis (FO) Performance
Kerusha Lutchmiah1,2, D.J.H. Harmsen1, B. Wols1, A.R.D. Verliefde2,3, J.W. Post1and E.R. Cornelissen1 Introduction
Conclusions
Literature
Cornelissen, E. R., Harmsen, D. J. H., Beerendonk, E. F., Qin, J. J., and Kappelhof, J. W. M. N. (2011). "Effect of draw solution type and operational mode of forward osmosis with lab-scale membranes and a spiral wound membrane module." Journal of Water Reuse and Desalination, 01(3), 133-140.
Hancock, N. T., Xu, P., Heil, D. M., Bellona, C., and Cath, T. Y. (2011). "Comprehensive bench- and pilot-scale investigation of trace organic compounds rejection by forward osmosis." Environmental Science and Technology, 45(19), 8483-8490. Xu, Y., Peng, X., Tang, C. Y., Fu, Q. S., and Nie, S. (2010). "Effect of draw solution concentration and operating conditions on forward osmosis and pressure retarded osmosis performance in a spiral wound module." Journal of Membrane Science, 348(1-2), 298-309.
• PAO might increase FO performance; however, the effect of PAO will be dependent on the type, structure and material of FO membranes.
• Further work focuses on the effects of (i) PAO on reverse salt transport and (ii) the effect of continuous PAO and discontinuous PAO operation (results not included).
• Possible mechanical breakdown due to additional feed pressure and pulsation might be an issue.
KWR Watercycle Research Institute,
P.O. box 1072, 3430 BB, Nieuwegein, The Netherlands For more information: emile.cornelissen@kwrwater.nl
Forward osmosis (FO) is a concentration-driven membrane process using an osmotic solution. Advantages of FO:
• low energy consumption • high product quality and
• lower fouling propensity compared to state-of-the-art pressure-driven membrane processes i.e. reverse osmosis
Typical fluxes obtained for spiral wound FO elements are 5-7 L/m2h for a 0.5 M NaCl osmotic draw solution (Cornelissen et al. 2011; Hancock et al. 2011; Xu et al. 2010); active layer (AL) facing the feed side (FS).
Materials and methods
• Reduce internal concentration polarisation(a limitation of FO) • Increase FO membrane performance (i.e. increase water flux,
decrease reverse salt flux)
Objectives of the research
2. PAO lab-scale experiments
PAO laboratory experiments were carried out in an FO set-up (Fig. 2). • Membrane: HTI Explorer type FO membrane (area = 112 cm2)
• FS: DI-water, DS: 0.5 M NaCl
• applied pressure range: 0-1 bar (only on the FS)
Results
1KWR, PO box 1072, 3430 BB Nieuwegein, The Netherlands
2Delft University of Technology, Stevinweg 1, 2628CN, Delft, The Netherlands 3Ghent University, Coupure Links 653, 9000 Ghent, Belgium
This study uses additional hydraulic pressure on the FS to investigate PAO, which aims to:
1. Modeling of water and solute flux in PAO
• ODE obtained from the solute transport equation in the membrane support layer (SL) (Figure 1).
• FO water flux (Jw) based on diffusion and additional hydraulic pressure difference (∆P) over the membrane by increasing the feed pressure (PF):
(1) A: water transport coefficient
πi: osmotic pressure at interface of AL and the membrane SL πF: osmotic pressure of FS
• Solving the ODE using the boundary conditions for the AL-FS, the following equation can be obtained for the solute concentration at the interface of the AL and the SL in PAO mode:
(2) B: solute transport coefficient
πF: osmotic pressure of the draw solution
Jw,FO: water flux without additional hydraulic pressure
Km: mass transport coefficient (depends on the structural parameter).
• FO flux increased with an increase in the feed pressure for both the PAO model and experiments.
• Experimentally, the FO flux increased significantly (<50%) when an additional feed pressure was applied (Figure 3). This is contradictory to calculated FO flux values which only increased slightly.
• Pressurising FO membranes can possibly change the water transport coefficient (A). This was not considered in the transport model, but will depend on the type, structure and material of the FO membrane.
P
FFigure 1: Concentration profiles in an FO membrane including additional hydraulic pressure at the feed side (PAO).
Figure 2. The lab-scale setup for PAO experiments.
F AP ) F π i A(π w J = − + + + π + π = B ) J A ).( J / (J B A ). J / (J ln K w J FO w, F FO w, w D FO w, w m
Figure 3: The effect of PAO on the water flux. Experimental values (squares) and calculated values (dots) are shown (A=1.29.10-12 m/Pa.s; B=4.12.10-8 m/s and S=6.10-4 m)
