2.1. Fundamentals of Forward Osmosis Process
2.1.1. Forward osmosis membranes
FO membranes are asymmetric in nature that consists of a dense active layer (AL) and a porous support layer (SL). The dense AL (~100 nm) is responsible for salt rejection, whereas the SL (~200 µm) provides the required mechanical strength to the membrane. Ideally, the FO system requires a semi-porous membrane capable of separating disintegrated solute species from the FS, high solute rejection, high water permeability, and superior chemical and mechanical stability [22].
In the FO process, membranes can be used either with the active layer (AL) facing FS (ALFS or FO-mode) or with the active layer facing DS (ALDS or PRO-mode) [36]. Zhao et al. (2011) [67] investigated the effect of membrane orientation on the FO performance of various applications. Reportedly, the selection of membrane orientation is influenced by the FS composition and the degree of concentration (or water recovery). Depending on the application, the AL-FS membrane orientation is preferred for the treatment of high-saline water, whereas the AL-DS membrane orientation is preferred for treating saline waters (such as brackish water) [68]. In both the membrane orientation, the net driving force is significantly less than the theoretical driving force. The reduction in net driving is apparently due to concentration polarisation.
CP refers to the accumulation of solutes near the membrane surface, which develops due to concentration differences at the membrane-solution interface [69]. Depending upon the nature of solute accumulation, the CP can be categorised as external concentration polarisation (ECP) and internal concentration polarisation (ICP) [70]. ICP occurs on the internal surface of the membrane (i.e., SL), whereas ECP occurs on the external surface of the membrane (i.e., AL).
The membrane orientation affects the relative solute accumulation in the SL or AL of the membrane, which gives rise to either dilutive or concentrative ICP or ECP [71]. In ALFS mode, the concentration of DS is higher near the external surface (AL), while the FS becomes less concentrated near the internal surface (SL), contributing to the concentrative ECP (CECP) and dilutive ICP (DICP), respectively. Similarly, for ALDS membrane orientation, concentrative ICP (CICP) and dilutive ECP (DECP) occurs [72].
Reportedly, the membrane orientation in the FO process poses a critical impact on ICP, which dominates the water flux decline. Thus, the impact of both ICP and ECP needs to be reduced to achieve a greater water flux. The section summarises the research advances aimed at
Chapter 02
overcoming the problem of concentration polarisation in the FO process. As ECP occurs on the external surface (AL) of the membrane, thus this phenomenon can be controlled by increasing the flow rate of the FS and DS streams. Gruber et al. (2011) [73] claimed that increasing the flow rate equalizes the concentration across the membrane surface, thus reducing the ECP effect.
Unlike ECP, the ICP takes place within the porous SL of the FO membrane. It is considered one of the most troublesome phenomena in FO processes because it cannot be easily eliminated. Therefore, the decline in water flux in FO is primarily caused by ICP, and the percentage of flux reduction can be as high as 80% [74]. In an asymmetric membrane, the ICP plays a significant role in reducing the osmotic pressure. The severity of ICP is influenced mainly by the structure of the SL [75]. The term structural parameter (S) is an intrinsic membrane parameter and describes the structure of SL of the FO membrane. Reportedly, high structural parameter decreases water flux due to increased mass resistance and ICP-effect (refer to equation 1.1).
In an ideal FO membrane, the SL should be thin, highly porous, and minimally tortuous.
Shaffer et al. (2015) [17] highlighted two prominent goals for membrane design: (i) minimising the structural parameter of the support layer (to mitigate mass transfer limitations and increase water flux) and (ii) maximising the reverse solute flux selectivity of the active layer (to limit the loss of draw solute). Thus, it suggests that to tackle ICP-effect, the SL should be more porous, but at the same time, to minimize RSF, the selectivity of the AL should be enhanced.
However, increasing membrane selectivity reduces water flux, which simultaneously induces RSF and fouling afterward.
Based on the materials used, the membranes used for FO application can be categorized as, Cellulose acetate ‘CA’ (or cellulose triacetate ‘CTA’), thin-film composite (TFC), and biomimetic membranes. TFC membranes comprise a dense AL with high salt rejection and a loosely bound support layer with high-mechanical strengths. Compared to other types of FO membranes, TFC membranes are more popular due to their high permeability, stability, chemical stability (pH 2-6), and design options [70]. However, most TFC membrane suffers from the ICP phenomenon, RSF, and to a certain extent, they are more prone to fouling due to their polyamide surfaces, which is part of their AL) [40].
Reportedly, till 2018 nearly all lab-scale FO experiments were conducted using flat-sheet FO membranes, of which 20% were self-manufactured and 76% were commercial membranes.
Literature review and objectives
Among these, almost 57% of the FO membranes were supplied by Hydration Technology Innovations (HTI, Albany, USA) [33]. Table 2.1 briefly overview some commercially available FO membranes.
Table 2.1 Overview of commercial forward osmosis membrane suppliers Supplier/ Manufacturer Membrane
type
Configuration Structural parameter (µm) Ref.
Aquaporin A/S (Copenhagen, Denmark)
TFC Hollow fiber 210 [76,77]
Flat sheet 630 [78]
OASYS Water Inc.
(Boston, MA, USA)
TFC Hollow fiber 550 [79]
Porifera Inc.
(Hayward, CA, USA
TFC Spiral wound 344 [80]
Toyobo Co. Ltd.
(Osaka, Japan)
CTA Hollow fiber 1024 [26]
Hydration Technology Innovations, HTI (USA)
CTA Flat sheet 663 [16,81]
TFC Flat sheet 1227
Fluid Technology solutions, FTS (USA)
CTA Spiral wound 707 [80]
TFC: thin-film composite; CTA: Cellulose triacetate
The earliest commercial development of FO technology was reported by Hydration Technology Innovations (HTI) in 1975 for emergency potable water supply for the US military.
In 2008, HTI patented commercial asymmetric CA/CTA FO membranes composed of a thin skin layer for salt separation (10–20 µm) and a thicker porous scaffold layer (about 100 µm thick) with a woven or non-woven mesh embedded within it [22]. In 2010, OASYS (Osmotic Applications and Systems) launched the world's first thin-film composite polyamide-based spiral wound (SW) FO membrane element. In 2014, Toyobo's hollow fiber CTA FO membrane demonstrated 10 times the area compared to the flat sheet membrane for seawater desalination and wastewater treatment. This membrane saves energy for seawater desalination as the need for a high-pressure pump and piping can be eliminated. The Aquaporin Inside flat sheet membrane launched the first commercially available thin-film composite FO membrane to incorporate aquaporin protein into its polyamide-based selective layer. Recently, Aquaporin
Chapter 02
Inside introduced its hollow fiber forward osmosis (HFFO) membrane module made of polyamide TFC. The manufacturer claims the membrane can reject difficult contaminants while preserving valuable components and offers a high-packing density.
Summary of FO membranes
Significant efforts have been made to improve the performance of FO membranes to achieve high water permeability (hydrophilicity) and high solute rejection ability, diminish the CP- effect (by reducing structural parameters and related membrane morphology), and, most notably, elevate the membrane stability (both chemical and mechanical stability) Figure 2.1 summarizes the critical factors that need to be considered while selecting FO membrane [16].
Although, most of the membranes mentioned above are developed for desalination and related applications. The detailed investigation of complex, viscous systems, such as liquid food extract, needs to be investigated. Thereby suggesting that there still exists scope for investigating a suitable FO membrane for concentrating liquid food extract.
Figure 2.1 Critical factor for selecting the ideal membranes for forward osmosis process