Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
Countries in Asia>
4. UPGRADING WATER QUALITY
Desalination provides a means of upgrading poor quality, saline waters,
and refers to a suite of technologies designed to produce freshwater from
saline sources such as seawater or brackish groundwater. There are
basically two types of desalination processes: reverse osmosis or RO
technologies and electro-dialysis technologies. The techniques are similar
in that both make use of membranes that allow passage of water molecules
but exclude salts and other contaminants, which are discharged as a brine
waste. In electro-dialysis, however, ions are transported through a
membrane from one solution to another under the influence of a direct
current electrical potential. The total concentration of soluble salts in
water is expressed in terms of either salinity (35 parts per thousand or
35 000 mg/l of total salts being standard seawater) or electrical
conductivity for diagnostic purposes. For example, saline water with
conductivity of less than 2 250 microSiemens/cm are suitable for
irrigation purposes. The major factors that determine which type of
treatment is best suited to a particular application include the levels of
salinity and temporary hardness; the presence of colloidal suspended
matter, dissolved metal ions, oxidizing agents, and hydrogen sulphide; and
temperature of the feed water, etc. (Crossley, 1983). The treatment of
saline water using this technology began in the 1940s and continues with
several hundred desalination plants operating throughout the Region. These
range in capacity from a few cubic metres to more than 10 000 cubic metres
per day (McRae, 1983). The largest plant built is in Baghdad, Iraq, which
desalinates 24 000 cubic metres of water per day (Khan, 1986). In some
areas of Asia, desalinization is vital to water supply.
The main components of a desalination plant consist of pretreatment and
post treatment storage areas, power supply, membrane stacks, membranes,
spacers, and pumps. Electro-dialysis plants also include electrodes. The
brackish water is passed over a stack of several hundred ion-permeable
membranes. In RO plants, the brackish water is passed across these
membranes under pressure, while, in electro-dialysis plants, the brackish
water is passed across these membranes at relatively low pressure as an
electrical current is passed across the membrane stack. During the latter
process, half the membranes allow positively charged ions to pass through
and half allow negatively charged ions to pass through. Typical recovery
rates using the electro-dialysis process range from 80% to 90% of the
volume of feed water (Frederick, 1992). This process uses energy at a rate
directly proportional to the quantity of salts to be removed and, for this
reason, is usually used for the treatment of brackish water. This
technology is very cost competitive for salinities up to 10 000 mg/l, but
is not well suited for desalinating high salinity seawater. These
technologies are relatively easy and simple to operate; are amenable to
rapid delivery and installation; have fast start up and shut down
capabilities; can be easily expanded to meet additional demand; and,
require a relatively small amount of available space for installation.
Small plants are often used for emergency water supply purposes.
Extent of Use
Desalination technologies were first conceived some 200 years ago and,
by the 1930's, several small desalination systems were constructed in the
Middle East. Since 1970, there has been significant commercial development
using various desalination technologies, including distillation, reverse
osmosis, and electrolysis, in the Middle East, where other options for
freshwater augmentation are limited. Desalination technologies have been
used to produce product waters with salinities below 500 mg/l (Khan,
1986). In Asia, a 9 072 m2 plant was built in Gwadar, Pakistan, to
desalinate sea water and, in Awania, India, a 1 867 m2 plant was built to
treat brackish water (Fenton, 1983). A United Nations study on the extent
of use of desalination technologies and their potential use in developing
countries, conducted in the early 1960s (Popkin, 1968), indicated
considerable potential for adoption of these technologies in Asia, in
India, Quatar, Saudi Arabia, Israel, and Kuwait.
Operation and Maintenance
The complexity of the operation and maintenance procedures for
desalination technologies depends upon the type of technology used.
Generally, however, an high level of expertise is required.
Level of Involvement
This technology has been implemented within the private sector at the
level of individual industries.
The costs of reverse osmosis (RO) and multistage flash (MSF)
desalination technologies have been tabulated by Tare et al.
(1991) and are shown in Table 13. As shown, RO appears to be the least
costly option for desalination, but, as their study concludes, the cost
effectiveness will largely depend on the local specific conditions. For
example, the cost of desalination using either technology will involve the
costs of withdrawal of water from the sea and of pumping the seawater the
plant. Nevertheless, in most cases, RO is usually the preferred means of
desalinating brackish water (rather than sea water):
- RO with multi-stage flash (MSF) is a reliable alternative
- - The capital cost of MSF is usually higher than RO due to high cost
of importing the equipment
- - At high capacities, the seawater intake volume is higher in MSF
plants, and efficiency lower
- - Power requirements are much higher in the case of RO plants because
of their high pressure operation
- - Though RO plants have lower initial investment and fixed costs,
they have higher maintenance costs
- - MSF plants require a greater land area, which can increase the cost
due to high land prices.
- TABLE 13. Comparison of Desalination Alternatives for Water
Supply to Industry in Arid Areas.
TABLE 14. Comparison of Alternative Industrial Water
Supply Technologies in Arid Areas.
|| Cost in $/1 000 m3
|Desalination of seawater
|| $73 - $1 621
|| $566 - $670
| Desalination of well water
|| $365 - $649
| Wastewater reclamation
|| $292 - $473
|| $162 - $567
TABLE 15. Component Costs of Production for MSF
Desalination Plants at Various Capacities.
TABLE 16. Component Cost of Production for RO
Desalination Plants at Various Capacities.
Effectiveness of the Technology
Using desalination technologies, water supply can be
increased incrementally with demand, by installing modular equipment in
stepwise fashion, whereas other technologies generally require that the
total investment be made at the beginning of the water resources
development project. Desalination technologies also can be quickly
installed, providing a rapid augmentation of freshwater supply without
conflicts over traditional water rights. On this basis, the US Department
of the Interior concluded that desalination is an entirely viable means of
increasing water supply, at least from the point of view of technological
feasibility (Clawson and Landsberg, 1972).
This technology is suitable for use in areas where
freshwater is scarce, but saline water is available and energy is cheap.
Advantages Compared to water recycling technologies, desalination presents
fewer health risks. The modular nature of this technology, using "packaged
plants", makes desalination useful in emergencies and for small-scale
Desalination is expensive. It is energy intensive and
incurs high energy costs. Its technical complexity may limit the ability
for the technology to be introduce without a major realignment of the
Desalination is a new technology, and requires further
testing before its cultural acceptability can be determined.
Further Development of the Technology
The application of desalination principles in a less
expensive and less complex form is being undertaken. For example, the
solar still is a low energy technology which has the future prospect of
providing inexpensive desalinated water. There is also the prospect of
using desalination technologies in combination with other technologies,
such as multistage flash evaporation, as a means of reducing the high
energy costs associated with the technology. The selection of this
technology to augment public water supply purposes depends on many
factors, including product water quality and quantity required; source
water characteristics, temperature and reliability of supply; energy
availability; and, the relative location of the consumers to the source.
Crossely, I.A. 1983. Desalination by Reverse Osmosis, In:
A. Porteous (ed.), Desalination Technology: Development and Practice,
Applied Science Publishers, London.
Fenton, G.G. 1983. Solar Distillation, In: A. Porteous
(ed.), Desalination Technology: Development and Practice, Applied
Science Publishers, London.
Khan, A.H. 1986. Desalination Processes and Multistage
Flash Distillation Practice. Elsevier, Amsterdam.
McRae, W.A. 1983. Electrodialysis, In: A. Porteous (ed.),
Desalination Technology: Development and Practice, Applied Science
Popkin, R .1968. Desalination: Water for the World's
Future, Frederick A. Praeger Publishers, New York.
Tare, M.M., et al. 1991. Economics of Desalination
in Water Resource Management - A Comparison of Alternative Water Resources
for Arid/Semi-arid Zones in Developing Countries, In: M. Balaban (ed.),
Proceedings (Wl.1) of the 12th International Symposium, Institute
of Chemical Engineering (UK), London.