Membrane and Desalination Technologies - TCE Moodle Website

560 J.P. Chen et al.
1. INTRODUCTION
Demand for potable water is increasing because of the reduction in reliable fresh water
sources and increase in global population. Therefore, it is necessary to search for technologies
that can convert non-conventional water sources into fresh water. Seawater is one such
abundant source which can be reached by most of the countries in the world. In order to
utilize seawater as potable water, it is required to remove the high salinity. Desalination
technologies are intended for the removal of dissolved salts that cannot be removed by
conventional treatment processes. Thermal distillation technologies have been used on
some ships for more than 100 years. Desalination was used on a limited scale for municipal
water treatment in the late 1960s. On the basis of desalination, the past four decades can be
divided into three phases: (1) 1950s were a time of discovery; (2) 1960s were concerned with
research; and (3) 1970s and 1980s were the time of commercialization. In the beginning of
the 1970s, the industry began to concentrate on commercially viable desalination applications
and processes.
The first commercial plant for the production of potable water from a saline source using
electrodialysis was put into operation in 1954. At that time, this process was not received
favorably because of its inability to reduce dissolved solids to a desired extent. The first
reverse osmosis (RO) water treatment plant was constructed in 1970s in Florida. Significant
advances in membrane materials and technologies in the last three decades have greatly
improved the cost effectiveness and performance capabilities of the processes. RO membrane
processes are increasingly used worldwide to solve a variety of water treatment problems.
In the desalination industry of USA, RO membrane technology is the most popular.
In this chapter, the process of RO and the mechanisms of membrane separation are
illustrated. Membrane materials are presented. Several mathematical models are given for
the membrane processes used in desalination. Pre- and posttreatment processes are discussed.
Examples, a case study, and recent developments in the seawater RO desalination technology
are elucidated.
2. MEMBRANE FILTRATION THEORY
2.1. Osmosis and RO
Osmosis is the phenomenon of water flow through a semipermeable membrane that blocks
the transport of salts or other solutes through the membrane. It is applied to water purification
and desalination, waste material treatment, and many other chemical and biochemical
processes. When two aqueous solutions (or other solvents) are separated by a semipermeable
membrane, water will flow from the side of low solute concentration to the side of high solute
concentration. The flow is stopped, or even reversed by applying external pressure on the side
of higher concentration. In such a case, the phenomenon is called RO, as shown in Fig. 13.1.
If there are solute molecules only on one side of the system, then the pressure that stops the
flow is called the osmotic pressure P, which is given by the Van’t Hoff equation:
P ¼ CSRT: (1Þ