Colloidal fouling in forward osmosis: Role of reverse salt diffusion
Graphical abstract
Highlights
► The important role of reverse salt diffusion in colloidal fouling of FO process is delineated. ► Draw solution type plays a key role in controlling the rate and extent of reverse salt diffusion. ► FO fouling with silica colloids exhibits reversibility after simple physical cleaning. ► Selection of proper draw solute is important for preventing reverse salt diffusion. ► Optimization of membrane selectivity is of paramount importance for efficient operation of FO.
Introduction
Global water shortages and depletion of fresh water supplies place a great demand on the development of alternative water sources, such as seawater desalination and wastewater reuse [1]. Improvements in membrane technology represent one of the most viable options to meet these critical challenges. Over the past few decades, innovative membrane technologies have been developed and applied to produce cleaner and safer water, including saline water desalination and ground/surface water treatment by reverse osmosis (RO) and nanofiltration (NF) [1], [2], wastewater treatment and reuse by membrane bioreactors [3], [4], and replacement of conventional water treatment systems by microfiltration (MF) and ultrafiltration (UF) [5], [6].
In particular, reverse osmosis (RO) technology has become the industry benchmark in membrane-based desalination and wastewater reuse because of its competitive cost and superior product water quality compared to conventional thermal and biological processes. Although RO technology has been vastly improved over the last few decades, its efficiency and sustainable operation are hampered by membrane fouling and by the considerable energy consumption for seawater desalination [1], [7], [8], [9], [10], [11], [12], [13]. Due to these limitations of RO technology, there is a demand for sustainable alternatives to conventional pressure-driven membrane processes.
For efficient and eco-friendly operation, future water treatment technologies should overcome the following limitations of conventional pressure-driven membrane processes: high energy consumption, demand for chemical cleaning agents to mitigate fouling, and considerable environmental impacts due to brine discharge. One technology that has the potential to become a sustainable alternative is forward osmosis (FO) [14], [15], [16], [17], [18]. In lieu of hydraulic pressure, FO utilizes a highly concentrated draw solution to induce the driving force for separation. The absence of hydraulic pressure in FO is expected to not only minimize the system energy consumption, but also to lower membrane fouling and the associated deleterious effects to the quality and quantity of the product water [19], [20].
In the FO process, a highly concentrated draw solution is placed opposite to the feed solution, which is separated by a semi-permeable membrane. The difference in the chemical potential across the membrane induces water flow from the feed to the draw solution side. However, this desired water flow is also accompanied by a concomitant transport of draw solutes to the feed side. This phenomenon of reverse diffusion of draw solutes to the feed solution is unavoidable in the current stage of FO technology. Recently, several attempts have been made to understand this mechanism and to quantify the amount of reverse diffusion of draw solutes [20], [21], [22], [23].
Since reverse transport of draw solutes is a mechanism unique to FO compared to conventional pressure-driven membrane processes, several phenomena that could be influenced by the transported draw solutes should be investigated. In particular, reverse salt diffusion may affect fouling behavior in FO, and therefore, have a significant impact on the efficiency of the process. While several recent studies investigated fouling of FO [24], [25], [26], very few studies addressed the effects of reverse solute diffusion on FO fouling [20], [26].
In this study, we investigate the impact of reverse draw solute diffusion on colloidal fouling behavior in the FO process. To better understand the role of reverse salt diffusion in FO fouling by colloidal silica particles, draw solutions which exhibit vastly different reverse diffusion rates, namely NaCl and LaCl3, were used. In addition to colloidal fouling rate, FO fouling reversibility was also investigated by employing different feed cross-flow velocities during the fouling runs, which is a common approach for fouling control in NF and RO. Based on the results, important mechanisms and factors that control colloidal fouling in FO are delineated and discussed.
Section snippets
Colloidal particle foulants
Two types of commercial silica particles were used as model foulants for the fouling experiments. The manufacturer (Nissan Chemical Industries, Ltd.) reports that these particles, denoted as ST-30 and ST-ZL, have diameters ranging from 10 to 20 nm and 70 to 140 nm, respectively. Our dynamic light scattering (ALV-5000, Langen, Germany) measurements, however, revealed that the intensity averaged diameters of the ST-30 and ST-ZL suspensions are 24 ± 5 and 139 ± 5, respectively. According to the
Characteristics of membrane and colloidal particles
The membrane water and salt (NaCl) permeability coefficients A and B were determined to be 2.81 × 10−12 m/(s Pa) and 4.66 × 10−8 m/s, respectively. The water permeability coefficient was lower than that reported for other cellulose-based membranes from HTI [25], [27]. The salt permeability coefficient was lower by more than a factor of two than that reported for FO membranes from HTI [27].
A transmission electron microscopy (TEM) image of the polydisperse silica suspension (i.e., a mixture of the small
Conclusion
This study demonstrates the importance of reverse salt diffusion in colloidal fouling and fouling reversibility in the FO process. Significant flux decline rates were observed with thick or less porous fouling layers composed of large (139 nm) particles or a mixture of particles (24 and 139 nm), respectively. The back diffusion of salts that permeated from the draw solution was hindered by the colloidal fouling layer and, consequently, the salt concentration near the membrane surface dramatically
Acknowledgements
This research was supported by the World Class University (WCU) program (Case III) through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R33-10046).
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