Standard methodology for evaluating membrane performance in osmotically driven membrane processes

TLDR: In this article, the authors present a standard methodology for testing of ODMP membranes based on experience gained and operating conditions used in forward osmosis (FO) and pressure-retarded osmo-sis (PRO) studies.

About: This article is published in Desalination.The article was published on 2013-03-01 and is currently open access. It has received 379 citations till now. The article focuses on the topics: Membrane permeability & Membrane.

Figures

Fig. 4. (a) Salt permeability coefficients and (b) salt rejections measured for the two ODMPmembranes during RO tests with 2000 mg/L NaCl feed water. Operating conditions: feed pressure 8.62 bar (125 psi), feed temperature 20 °C, feed flow velocity 0.25 m/s.

Table 1 Testing conditions reported in recent publications on FO membrane development.

Fig. 6. Reverse salt flux during (a) FO and (b) PRO tests with two ODMP membranes. Operating conditions as detailed in Table 2 and in the caption of Fig. 5.

Fig. 7. Structural parameters of the two ODMPmembranes. Operating conditions as detailed in Table 2 and in the caption of Fig. 5.

Fig. 1. Water flux as a function of draw solution concentration for different combinations of feed and draw solution temperatures. FO experiments were conducted with first generation HTI membrane (imbedded mesh) oriented with the support layer facing the draw solution (FO mode), no spacers in the flow channels, deionized water feed, and flow velocity of 0.2 m/s [47].

Table 2 Operating conditions for testing of ODMP membranes in FO, PRO, and RO modes.

Frequently asked questions

Q1. What have the authors contributed in "Standard methodology for evaluating membrane performance in osmotically driven membrane processes" ?

In this paper, a standard method for characterizing forward osmosis membranes is proposed and results from a round-robin test of the method by seven independent laboratories are presented. 

Q2. Why do different laboratories use different membrane cells?

Because different laboratories use different membrane cells, feed and draw solution flow rates have to be adjusted to achieve predetermined cross-flow velocity, which has direct influence on the mixing and mass transfer in the flow channel. 

Q3. Why is the draw solution solutes concentrated in the feed stream during the FO and PRO experiments?

Because of the bi-directional diffusion of solutes during osmosis, draw solution solutes will slowly accumulate in the feed stream during FO and PRO experiments, and they may get further concentrated due to permeation of feed water into the draw solution (dewatering of the feed stream). 

Q4. What is the common method of testing a membrane in PRO mode?

When tested in PROmode for the purpose of quantifying water and solute flux, the membrane is turned over with the support layer in contact with the feed stream andwith no transmembrane pressure. 

Q5. What are the main reasons for the rapid development of ODMPs?

Increasing demand for water and electricity and encouragingresults from ODMP studies in recent years, have accelerated the development of ODMP technologies, including a flurry of membrane development work [5–21] intended to enable further improvement and commercialization of ODMPs. 

Q6. What is the effect of the draw solution on the osmotic pressure?

While the composition and concentration of the draw solution solutes dictate its osmotic pressure, the actual driving force induced by the draw solution is also controlled by the viscosity of solution and diffusivity of the solutes [35]. 

Q7. What is the common mode of membrane testing?

When tested in FO mode, the membrane is always oriented with its active layer in contact with the feed stream/solution and the porous support layer in contact with the draw solution. 

Q8. How many m/s is the flow velocity selected for the feed and draw solutions?

Based on the operating conditions and dimensions of most ODMP test cells, 0.25 m/s is the flow velocity that was selected for the feed and draw solutions in the round-robin study and for the proposed methodology. 

Q9. How can The authormeasure the permeation rate in a membrane cell?

Permeation rate can bemeasured by collecting permeate in a graduate cylinder, by measuring it with a rotameter or an electronic flow meter, or by collecting the permeate in a beaker placed on an analytical balance. 

Q10. What is the flow rate in a ODMP test cell?

Like in ODMP test systems, feed and permeate concentrations must be continuously or intermittently monitored and feed temperature, pressure, and flow velocity strictly controlled. 

Q11. What is the range of pH tolerance of ODMP membranes?

first generation ODMPmembranes aremade of cellulose acetate, which has a relatively narrow range of pH tolerance, usually between 4 and 8 [43]. 

Q12. What is the importance of ensuring that the transmembrane pressure across the membrane approaches?

it is also important to ensure that the transmembrane pressure across the membrane during testing of ODMP membranes approaches zero. 

Q13. What is the importance of the characterization of themembrane properties?

accurate characterization of themembrane properties is critical to the prediction of OMDP performance under given operation conditions. 

Q14. Why are ODMP membranes more delicate than commercial RO and NF?

Membranes for ODMP might be more delicate than commercial RO and NF membranes because in some ODMP the operating hydraulic pressure is much lower andmechanical support is not a major concern. 

Q15. What can be inferred from Eqs. (1)–(5)?

From Eqs. (1)–(5) it can be inferred that both operating conditions and membrane properties can substantially affect process performance. 

Q16. How can The authordetermine the flow rate in a flow channel?

The flow rate in each channel can be determined bymultiplying the predetermined cross-flowvelocity by the cross sectional area of the flow channel perpendicular to the flow direction. 

Q17. How high should the feed pressure be during ODMP testing?

Based on the performance and limitations of current ODMP membranes, the authors have suggested that the feed pressure during RO testing of ODMPmembranes be limited to 8.62 bar (125 psi). 

Q18. What is the relationship between the water and reverse salt flux?

This relationship indicates that the ratio of the water and reverse salt flux is a function of the transport properties of themembrane active layer and is independent of the structure of the support layer. 

Q19. What is the effect of the temperature on the membrane?

Results in Fig. 1 demonstrate that water flux in FO is strongly affected by the temperatures on both sides of the membrane and by draw solution concentrations.