Effects of membrane orientation on process performance in forward osmosis applications
Highlights
► The effects of membrane orientation on FO performance are investigated. ► Selection of the orientation is determined by the occurrence of the isoflux point. ► FO mode provides better performance when severe fouling occurs. ► It provides a guideline for the orientation selection in various FO applications.
Introduction
As an emerging technology forward osmosis (FO) has attracted growing interest in seawater/brackish water desalination [1], [2], [3], wastewater treatment [4], [5], [6], liquid food processing [7], [8], [9], [10] and power generation [11], [12], [13], [14]. FO is an osmotically driven membrane process that forces water across a semipermeable membrane from the feed solution with low osmotic pressure to the draw solution with high osmotic pressure. Low hydraulic pressure is required in FO, so it offers great opportunities for energy saving and even power generation [13], [15].
Currently the only commercially available FO membrane (from Hydration Technology Innovations) and most of the laboratory-prepared flat sheet membranes [16], [17], [18] are a type of asymmetric/composite membrane with a dense active layer and a porous support layer. The FO performances are completely different when the feed solution is placed against the two different layers of the membrane [19]. Here we refer the membrane orientation as pressure retarded osmosis (PRO) mode when the feed solution is facing the support layer, and as FO mode when the feed is facing the active layer.
It has been demonstrated that the membrane orientation in FO poses critical impacts on internal concentration polarization (ICP), which dominates the water flux decline [19], [20]. Recently Tang's group showed that membrane fouling was influenced by the membrane orientation using latex particles as model foulant [21]. Additionally they found that the boron flux was also affected by the membrane orientation and a greater boron flux was observed in PRO mode compared to the other membrane orientation [22]. Both PRO and FO modes have been used in various FO applications. The membrane orientations used in these FO applications are summarized in Table 1. However, there is still no unanimous agreement on the selection of membrane orientation in these applications.
The objective of this work is to investigate the effects of membrane orientation on FO performance in various applications. Both permeate flux and reverse solute flux were studied in PRO and FO modes. The effect of membrane orientation on FO performance was systematically investigated under the conditions of no fouling, with inorganic fouling (scaling) and organic fouling. The membrane fouling tendency and cleaning efficiency were compared in PRO and FO modes. The results have significant implications for the selection of membrane orientation for specific FO application.
Section snippets
FO membrane
Flat sheet cellulose triacetate (CTA) FO membranes (Hydration Technologies, Albany, OR) were used in the FO experiments. These membranes are unique compared to other semipermeable membranes (e.g. RO membranes), and have been determined to be the best available membranes for current FO applications [2], [3].
Feed and draw solutions
Analytical reagent grade NaCl (Ajax Finechem Pty. Ltd., Australia) was used as draw solute in all of FO experiments. Saline water without and with inorganic or organic foulants was used as
Membrane performance in PRO and FO modes
Mass transfer (i.e. the water flux and the reverse solute flux) in both PRO and FO modes as a function of draw solution concentration is depicted in Fig. 2. It can be seen that both the water flux and the reverse solute flux increase almost linearly with the increase of the draw solution concentration due to no fouling and low concentration polarization at the relatively low concentrations. However, both the water flux and the solute flux in PRO mode are obviously higher than that in FO mode,
Conclusions
In this work, the effects of membrane orientation on FO performance in saline water desalination without fouling, and with inorganic or organic fouling were systematically investigated through using a bench-scale membrane system. It is found that the selection of membrane orientation is basically influenced by the feed solution composition and the concentration degree (i.e. concentration factor or water recovery). When severe membrane fouling or scaling occurs, the isoflux point is arrived
Acknowledgements
The authors thank China Scholarship Council (CSC) and University of South Australia for providing the PhD scholarships. Shuaifei Zhao also would like to express his appreciation to Ms. Sara Azari for her advice and discussion.
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