Elsevier

Desalination

Volume 278, Issues 1–3, 1 September 2011, Pages 157-164
Desalination

Effects of working temperature on separation performance, membrane scaling and cleaning in forward osmosis desalination

https://doi.org/10.1016/j.desal.2011.05.018Get rights and content

Abstract

Recently forward osmosis (FO) has drawn increasing attention in wastewater treatment, brackish water/seawater desalination and power generation. It is supposed that FO has many advantages over other pressure-driven processes, such as low energy consumption, low fouling tendency, high water recovery and thus minimizing brine volume. In FO process, many parameters like osmotic pressure, fluid viscosity, mass transfer and mineral solubility are temperature dependent. In the current study, the effects of working temperature on separation performance (e.g. water fluxes and recoveries), membrane scaling and cleaning were systematically investigated through a bench-scale FO system. Both real and simulated brackish water were used as the feed solution in FO at temperatures of 25, 35 and 45 °C. Bench-scale FO experiments showed that higher temperature would afford higher initial fluxes, higher water recoveries and higher concentration factors, but also caused more adverse effects on membrane scaling and cleaning.

Highlights

► Forward osmosis was used for bench scale brackish water desalination experiment. ► Effects of temperature on separation performance, membrane scaling and cleaning were studied. ► Higher temperature resulted in higher initial fluxes and higher water recoveries. ► More adverse effects on membrane scaling and cleaning were also caused at higher temperature.

Introduction

A variety of desalination technologies both thermally-driven and membrane-based, have been increasingly employed to enhance the limited freshwater supply. Among them reverse osmosis (RO) is regarded as the most economical and popular desalination way for water production mainly due to the advancement of membrane technology [1]. However, RO is still hampered by at least three key obstacles: membrane fouling, high energy consumption and limited water recovery [2]. Typically energy inputs can account for 44% of the total water costs of a RO plant [3]. While the water recoveries of single-stage desalination systems range from 40% to 60%, although second-stage RO can further increase the water recovery with an added energy and capital costs. Limited water recovery causes large volume of concentrated brine, which has become a critical environmental concern [4], especially for inland brackish water desalination plants where brine discharging sources (e.g. sea) are unavailable. Another critical problem for brackish water desalination is membrane scaling (or inorganic fouling) caused by salt precipitation [1], which have been intensively studied [5], [6], [7], [8].

With the advantages of low energy costs and low membrane fouling, forward osmosis (FO) has attracted growing interest in various water treatment processes [4], [9], [10], [11], [12]. In a FO process, water diffuses spontaneously through a semi-permeable membrane from a feed solution (FS) with low osmotic pressure to a draw solution (DS) with high osmotic pressure. The driving force in FO which is from the osmotic pressure difference between the draw solution and the feed solution can be much greater than the hydraulic driving force in RO. This will potentially lead to higher water fluxes and recoveries. Since no or low hydraulic pressure is applied, lower energy and thus less cost are required in FO, compared with RO. Additionally, a more recent study [13] shows that FO can be used as an osmotic dilution process for seawater desalination and impaired water purification in a hybrid FO/RO process. In that process, FO can provide several major benefits including high quality of drinking water due to the multi-barrier protection, low energy input and reduced RO fouling because of pre-treatment by FO.

An important factor in membrane desalination processes is temperature, which is related to mass transfer [14], mineral solubility [15], membrane fouling [8], [16], [17] and concentration polarization [18]. In RO, the effects of feed temperature on separation performance have been investigated by many researchers [8], [14], [17], [18]. However, there is no reported literature on the effects of operation temperature on FO performance even though temperature is also of great significance in FO.

The objective of this study is to investigate the effects of operation temperature on separation performance and membrane scaling in FO desalination. Three temperatures (25, 35 and 45 °C) were selected and their effects on separation performance (e.g. water fluxes and recoveries), membrane scaling and cleaning were systematically investigated through a bench-scale FO system.

Section snippets

Theory

In membrane processes, many phenomena such as mass transfer, concentration polarization and membrane fouling are temperature dependent.

FO membranes and characterization

Flat-sheet cellulose triacetate (CTA) FO membranes (Hydration Technologies, Albany, OR) were used in the FO experiments. These membranes are unique compared with other semi-permeable membranes (e.g. RO membranes), and have been reported to be the best available membranes for current FO applications [4], [9].

The water contact angles of the FO membrane selective (or active) layer and back (support) layer were (61.3° ± 0.8°) and (66.4° ± 1.3°), respectively, measured by a DataPhysics OCA15 Contact

FO membrane characterization

The microstructure of the FO membrane was determined by FEG-SEM. The SEM images of the membrane (Fig. 4) show that the microstructure of the FO membrane is distinctively different from that of the conventional thin film composite RO membrane which generally has three layers: a thick non-woven fabric layer, a microporous layer and a dense selective layer [30], [31]. In Fig. 4, the SEM image of the selective layer shows that a lot of ridges and valleys uniformly exist on the selective layer,

Conclusions

This study investigated the effects of working temperature on FO separation performance, membrane scaling and cleaning in brackish water desalination. As temperature increased from 25 to 45 °C, FO separation performance in terms of initial permeate fluxes and final recoveries was significantly enhanced. Bench-scale FO experiments showed that higher temperature would afford higher initial permeate fluxes, higher water recoveries and higher concentration factors, but also caused more adverse

Acknowledgments

The authors thank China Scholarship Council (CSC) and University of South Australia for providing the PhD scholarships.

References (32)

Cited by (193)

View all citing articles on Scopus
View full text